An Engineer’s Guide to App Metrics

Building and shipping a successful product takes more than raw engineering. I have been posting a bit about using Telemetry to learn about how people interact with your application so you can optimize use cases. There are other types of data you should consider too. Being aware of these metrics can help provide a better focus for your work and, hopefully, have a bigger impact on the success of your product.

Active Users

This includes daily active users (DAUs) and monthly active users (MAUs). How many people are actively using the product within a time-span? At Mozilla, we’ve been using these for a long time. From what I’ve read, these metrics seem less important when compared to some of the other metrics, but they do provide a somewhat easy to measure indicator of activity.

These metrics don’t give a good indication of how much people use the product though. I have seen a variation metric called DAU/MAU (daily divided by monthly) and gives something like retention or engagement. DAU/MAU rates of 50% are seen as very good.


This metric focuses on how much people really use the product, typically tracking the duration of session length or time spent using the application. The amount of time people spend in the product is an indication of stickiness. Engagement can also help increase retention. Mozilla collects data on session length now, but we need to start associating metrics like this with some of our experiments to see if certain features improve stickiness and keep people using the application.

We look for differences across various facets like locales and releases, and hopefully soon, across A/B experiments.

Retention / Churn

Based on what I’ve seen, this is the most important category of metrics. There are variations in how these metrics can be defined, but they cover the same goal: Keep users coming back to use your product. Again, looking across facets, like locales, can provide deeper insight.

Rolling Retention: % of new users return in the next day, week, month
Fixed Retention: % of this week’s new users still engaged with the product over successive weeks.
Churn: % of users who leave divided by the number of total users

Most analysis tools, like iTunes Connect and Google Analytics, use Fixed Retention. Mozilla uses Fixed Retention with our internal tools.

I found some nominal guidance (grain of salt required):
1-week churn: 80% bad, 40% good, 20% phenomenal
1-week retention: 25% baseline, 45% good, 65% great

Cost per Install (CPI)

I have also seen this called Customer Acquisition Cost (CAC), but it’s basically the cost (mostly marketing or pay-to-play pre-installs) of getting a person to install a product. I have seen this in two forms: blended – where ‘installs’ are both organic and from campaigns, and paid – where ‘installs’ are only those that come from campaigns. It seems like paid CPI is the better metric.

Lower CPI is better and Mozilla has been using Adjust with various ad networks and marketing campaigns to figure out the right channel and the right messaging to get Firefox the most installs for the lowest cost.

Lifetime Value (LTV)

I’ve seen this defined as the total value of a customer over the life of that customer’s relationship with the company. It helps determine the long-term value of the customer and can help provide a target for reasonable CPI. It’s weird thinking of “customers” and “value” when talking about people who use Firefox, but we do spend money developing and marketing Firefox. We also get revenue, maybe indirectly, from those people.

LTV works hand-in-hand with churn, since the length of the relationship is inversely proportional to the churn. The longer we keep a person using Firefox, the higher the LTV. If CPI is higher than LTV, we are losing money on user acquisition efforts.

Total Addressable Market (TAM)

We use this metric to describe the size of a potential opportunity. Obviously, the bigger the TAM, the better. For example, we feel the TAM (People with kids that use Android tablets) for Family Friendly Browsing is large enough to justify doing the work to ship the feature.

Net Promoter Score (NPS)

We have seen this come up in some surveys and user research. It’s suppose to show how satisfied your customers are with your product. This metric has it’s detractors though. Many people consider it a poor value, but it’s still used quiet a lot.

NPS can be as low as -100 (everybody is a detractor) or as high as +100 (everybody is a promoter). An NPS that is positive (higher than zero) is felt to be good, and an NPS of +50 is excellent.

Go Forth!

If you don’t track any of these metrics for your applications, you should. There are a lot of off-the-shelf tools to help get you started. Level-up your engineering game and make a bigger impact on the success of your application at the same time.

Fun With Telemetry: URL Suggestions

Firefox for Android has a UI Telemetry system. Here is an example of one of the ways we use it.

As you type a URL into Firefox for Android, matches from your browsing history are shown. We also display search suggestions from the default search provider. We also recently added support for displaying matches to previously entered search history. If any of these are tapped, with one exception, the term is used to load a search results page via the default search provider. If the term looks like a domain or URL, Firefox skips the search results page and loads the URL directly.


  1. This suggestion is not really a suggestion. It’s what you have typed. Tagged as user.
  2. This is a suggestion from the search engine. There can be several search suggestions returned and displayed. Tagged as engine.#
  3. This is a special search engine suggestion. It matches a domain, and if tapped, Firefox loads the URL directly. No search results page. Tagged as url
  4. This is a matching search term from your search history. There can be several search history suggestions returned and displayed. Tagged as history.#

Since we only recently added the support for search history, we want to look at how it’s being used. Below is a filtered view of the URL suggestion section of our UI Telemetry dashboard. Looks like history.# is starting to get some usage, and following a similar trend to engine.# where the first suggestion returned is used more than the subsequent items.

Also worth pointing out that we do get a non-trivial amount of url situations. This should be expected. Most search keyword data released by Google show that navigational keywords are the most heavily used keywords.

An interesting observation is how often people use the user suggestion. Remember, this is not actually a suggestion. It’s what the person has already typed. Pressing “Enter” or “Go” would result in the same outcome. One theory for the high usage of that suggestion is it provides a clear outcome: Firefox will search for this term. Other ways of trigger the search might be more ambiguous.


Is Ad Blocking Really About The Ads?

Since Apple released iOS9 with Content Blocking extensions for Safari, there has been a lot of discussion about the ramifications. On one side, you have the content providers who earn a living by monetizing the content they generate. On the other side, you have consumers who view the content on our devices, trying to focus on the wonderful content and avoid those annoying advertisements.

But wait, is it really the ads? The web has had advertisements for a long time. Over the years, the way Ad Networks optimize the efficiency of ad monetization has changed. I think it happened slowly enough that we, as consumers, largely didn’t noticed some of the downsides. But other people did. They realized that Ad Networks track us in ways that might feel like an invasion of privacy. So those people started blocking ads.

Even more recently, we’ve started to discover that the mechanisms Ad Networks use to serve and track ad also create horrible performance problems, especially on mobile devices. If pages load slowly in a browser, people notice. If pages start consuming more bandwidth, people notice. If you provide a way to quickly and easily improve performance and reduce data usage, people will try it. I’d even posit that people care a lot more about performance and data usage, than privacy.

Even if you don’t care about the privacy implications of tracking cookies and other technologies sites use to identify us online, you might want to turn on Tracking Protection in Firefox anyway for a potential big speed boost. – Lifehacker

Is there a way for Ad Networks to clean up this mess? Some people inside Mozilla think it can be done, but it will take some effort and participation from the Ad Networks. I don’t think anyone has a good plan yet. Maybe the browser could help Ad Networks do things much more efficiently, improving performance and reducing bandwidth. Maybe people could choose how much personal information they want to give up.

If you fix the performance, data usage and privacy issues – will people really care that much about advertisements?

Random Management: Unblocking Technical Leadership

I’m an Engineering Manager, previously a Senior Developer. I have done a lot of coding. I still try to do a little coding every now and then. Because of my past, I could be oppressive to senior developers on my teams. When making decisions, I found myself providing both the management viewpoint and the technical viewpoint. This usually means I was keeping a perfectly qualified technical person from participating at a higher level of responsibility. This creates an unhealthy technical organization with limited career growth opportunities.

As a manager with a technical background, I found it difficult to separate the two roles, but admitting there was a problem was a good first step. Over the last few years, I have been trying to get better at creating more room for technical people to grow on my teams. It seems to be more about focusing on outcomes for them to target, finding opportunities for them to tackle, listening to what they are telling me, and generally staying out of the way.

Another thing to keep in mind, it’s not just an issue with management. The technical growth track is a lot like a ladder: Keep developers climbing or everyone can get stalled. We need to make sure Senior Developers are working on suitable challenges or they end up taking work away from Junior Developers.

I mentioned this previously, but it’s important to create a path for technical leadership. With that in mind, I’m really happy about the recently announced Firefox Technical Architects Group. Creating challenges for our technical leadership, and roles with more responsibility and visibility. I’m also interested to see if we get more developers climbing the ladder.

Random Thoughts on Management

I have ended up managing people at the last three places I’ve worked, over the last 18 years. I can honestly say that only in the last few years have I really started to embrace the job of managing. Here’s a collection of thoughts and observations:

Growth: Ideas and Opinions and Failures

Expose your team to new ideas and help them create their own voice. When people get bored or feel they aren’t growing, they’ll look elsewhere. Give people time to explore new concepts, while trying to keep results and outcomes relevant to the project.

Opinions are not bad. A team without opinions is bad. Encourage people to develop opinions about everything. Encourage them to evolve their opinions as they gain new experiences.

“Good judgement comes from experience, and experience comes from bad judgement” – Frederick P. Brooks

Create an environment where differing viewpoints are welcomed, so people can learn multiple ways to approach a problem.

Failures are not bad. Failing means trying, and you want people who try to accomplish work that might be a little beyond their current reach. It’s how they grow. Your job is keeping the failures small, so they can learn from the failure, but not jeopardize the project.

Creating Paths: Technical versus Management

It’s important to have an opinion about the ways a management track is different than a technical track. Create a path for managers. Create a different path for technical leaders.

Management tracks have highly visible promotion paths. Organization structure changes, company-wide emails, and being included in more meetings and decision making. Technical track promotions are harder to notice if you don’t also increase the person’s responsibilities and decision making role.

Moving up either track means more responsibility and more accountability. Find ways to delegate decision making to leaders on the team. Make those leaders accountable for outcomes.

Train your engineers to be successful managers. There is a tradition in software development to use the most senior engineer to fill openings in management. This is wrong. Look for people that have a proclivity for working with people. Give those people management-like challenges and opportunities. Once they (and you) are confident in taking on management, promote them.

Snowflakes: Engineers are Different

Engineers, even great ones, have strengthens and weaknesses. As a manager, you need to learn these for each person on your team. People can be very strong at starting new projects, building something from nothing. Others can be great at finishing, making sure the work is ready to release. Some excel at user-facing code, others love writing back-end services. Leverage your team’s strengthens to efficiently ship products.

“A 1:1 is your chance to perform weekly preventive maintenance while also understanding the health of your team” – Michael Lopp (rands)

The better you know your team, the less likely you will create bored, passionless drones. Don’t treat engineers as fungible, swapable resources. Set them, and the team, up for success. Keep people engaged and passionate about the work.

Further Reading

The Role of a Senior Developer
On Being A Senior Engineer
Want to Know Difference Between a CTO and a VP of Engineering?
Thoughts on the Technical Track
The Update, The Vent, and The Disaster
Bored People Quit
Strong Opinions, Weakly Held

Firefox for Android: What’s New in v35

The latest release of Firefox for Android is filled with new features, designed to work with the way you use your mobile device.


Search is the most common reason people use a browser on mobile devices. To help make it easier to search using Firefox, we created the standalone Search application. We have put the features of Firefox’s search system into an activity that can more easily be accessed. You no longer need to launch the full browser to start a search.

When you want to start a search, use the new Firefox Widget from the Android home screen, or use the “swipe up” gesture on the Android home button, which is available on devices with software home buttons.


Once started, just start typing your search. You’ll see your search history, and get search suggestions as you type.


The search results are displayed in the same activity, but tapping on any of the results will load the page in Firefox.


Your search history is shared between the Search and Firefox applications. You have access to the same search engines as in Firefox itself. Switching search engines is easy.


Another cool feature is the Sharing overlay. This feature grew out of the desire to make Firefox work with the way you use mobile devices. Instead of forcing you to switch away from applications when sharing, Firefox gives you a simple overlay with some sharing actions, without leaving the current application.


You can add the link to your bookmarks or reading list. You can also send the link to a different device, via Firefox Sync. Once the action is complete, you’re back in the application. If you want to open the link, you can tap the Firefox logo to open the link in Firefox itself.

Synced Tabs

Firefox Sync makes it easy to access your Firefox data across your different devices, including getting to the browser tabs you have open elsewhere. We have a new Synced Tabs panel available in the Home page that lets you easily access open tabs on other devices, making it simple to pick up where you left off.

Long-tap an item to easily add a bookmark or share to another application. You can expand/collapse the device lists to manage the view. You can even long-tap a device and hide it so you won’t see it again!


Improved Error Pages

No one is happy when an error page appears, but in the latest version of Firefox the error pages try to be a bit more helpful. The page will look for WiFi problems and also allow you to quickly search for a problematic address.


Firefox for Android: Collecting and Using Telemetry

Firefox 31 for Android is the first release where we collect telemetry data on user interactions. We created a simple “event” and “session” system, built on top of the current telemetry system that has been shipping in Firefox for many releases. The existing telemetry system is focused more on the platform features and tracking how various components are behaving in the wild. The new system is really focused on how people are interacting with the application itself.

Collecting Data

The basic system consists of two types of telemetry probes:

  • Events: A telemetry probe triggered when the users takes an action. Examples include tapping a menu, loading a URL, sharing content or saving content for later. An Event is also tagged with a Method (how was the Event triggered) and an optional Extra tag (extra context for the Event).
  • Sessions: A telemetry probe triggered when the application starts a short-lived scope or situation. Examples include showing a Home panel, opening the awesomebar or starting a reading viewer. Each Event is stamped with zero or more Sessions that were active when the Event was triggered.

We add the probes into any part of the application that we want to study, which is most of the application.

Visualizing Data

The raw telemetry data is processed into summaries, one for Events and one for Sessions. In order to visualize the telemetry data, we created a simple dashboard (source code). It’s built using a great little library called PivotTable.js, which makes it easy to slice and dice the summary data. The dashboard has several predefined tables so you can start digging into various aspects of the data quickly. You can drag and drop the fields into the column or row headers to reorganize the table. You can also add filters to any of the fields, even those not used in the row/column headers. It’s a pretty slick library.


Acting on Data

Now that we are collecting and studying the data, the goal is to find patterns that are unexpected or might warrant a closer inspection. Here are a few of the discoveries:

Page Reload: Even in our Nightly channel, people seem to be reloading the page quite a bit. Way more than we expected. It’s one of the Top 2 actions. Our current thinking includes several possibilities:

  1. Page gets stuck during a load and a Reload gets it going again
  2. Networking error of some kind, with a “Try again” button on the page. If the button does not solve the problem, a Reload might be attempted.
  3. Weather or some other update-able page where a Reload show the current information.

We have started projects to explore the first two issues. The third issue might be fine as-is, or maybe we could add a feature to make updating pages easier? You can still see high uses of Reload (reload) on the dashboard.

Remove from Home Pages: The History, primarily, and Top Sites pages see high uses of Remove (home_remove) to delete browsing information from the Home pages. People do this a lot, again it’s one of the Top 2 actions. People will do this repeatably, over and over as well, clearing the entire list in a manual fashion. Firefox has a Clear History feature, but it must not be very discoverable. We also see people asking for easier ways of clearing history in our feedback too, but it wasn’t until we saw the telemetry data for us to understand how badly this was needed. This led us to add some features:

  1. Since the History page was the predominant source of the Removes, we added a Clear History button right on the page itself.
  2. We added a way to Clear History when quitting the application. This was a bit tricky since Android doesn’t really promote “Quitting” applications, but if a person wants to enable this feature, we add a Quit menu item to make the action explicit and in their control.
  3. With so many people wanting to clear their browsing history, we assumed they didn’t know that Private Browsing existed. No history is saved when using Private Browsing, so we’re adding some contextual hinting about the feature.

These features are included in Nightly and Aurora versions of Firefox. Telemetry is showing a marked decrease in Remove usage, which is great. We hope to see the trend continue into Beta next week.

External URLs: People open a lot of URLs from external applications, like Twitter, into Firefox. This wasn’t totally unexpected, it’s a common pattern on Android, but the degree to which it happened versus opening the browser directly was somewhat unexpected. Close to 50% of the URLs loaded into Firefox are from external applications. Less so in Nightly, Aurora and Beta, but even those channels are almost 30%. We have started looking into ideas for making the process of opening URLs into Firefox a better experience.

Saving Images: An unexpected discovery was how often people save images from web content (web_save_image). We haven’t spent much time considering this one. We think we are doing the “right thing” with the images as far as Android conventions are concerned, but there might be new features waiting to be implemented here as well.

Take a look at the data. What patterns do you see?

Here is the obligatory UI heatmap, also available from the dashboard:

Firefox for Android: Casting videos and Roku support – Ready to test in Nightly

Firefox for Android Nightly builds now support casting HTML5 videos from a web page to a TV via a connected Roku streaming player. Using the system is simple, but it does require you to install a viewer application on your Roku device. Firefox support for the Roku viewer and the viewer itself are both currently pre-release. We’re excited to invite our Nightly channel users to help us test these new features, share feedback and file any bugs so we can continue to make improvements to performance and functionality.


To begin testing, first you’ll need to install the viewer application to your Roku. The viewer app, called Firefox for Roku Nightly, is currently a private channel. You can install it via this link: Firefox Nightly

Once installed, try loading this test page into your Firefox for Android Nightly browser: Casting Test

When Firefox has discovered your Roku, you should see the Media Control Bar with Cast and Play icons:


The Cast icon on the left of the video controls allows you to send the video to a device. You can also long-tap on the video to get the context menu, and cast from there too.

Hint: Make sure Firefox and the Roku are on the same Wifi network!

Once you have sent a video to a device, Firefox will display the Media Control Bar in the bottom of the application. This allows you to pause, play and close the video. You don’t need to stay on the original web page either. The Media Control Bar will stay visible as long as the video is playing, even as you change tabs or visit new web pages.


You’ll notice that Firefox displays an “active casting” indicator in the URL Bar when a video on the current web page is being cast to a device.

Limitations and Troubleshooting

Firefox currently limits casting HTML5 video in H264 format. This is one of the formats most easily handled by Roku streaming players. We are working on other formats too.

Some web sites hide or customize the HTML5 video controls and some override the long-tap menu too. This can make starting to cast difficult, but the simple fallback is to start playing the video in the web page. If the video is H264 and Firefox can find your Roku, a “ready to cast” indicator will appear in the URL Bar. Just tap on that to start casting the video to your Roku.

If Firefox does not display the casting icons, it might be having a problem discovering your Roku on the network. Make sure your Android device and the Roku are on the same Wifi network. You can load about:devices into Firefox to see what devices Firefox has discovered.

This is a pre-release of video casting support. We need your help to test the system. Please remember to share your feedback and file any bugs. Happy testing!

Firefox for Android: Your Feedback Matters!

Millions of people use Firefox for Android every day. It’s amazing to work on a product used by so many people. Unsurprisingly, some of those people send us feedback. We even have a simple system built into the application to make it easy to do. We have various systems to scan the feedback and look for trends. Sometimes, we even manually dig through the feedback for a given day. It takes time. There is a lot.

Your feedback is important and I thought I’d point out a few recent features and fixes that were directly influenced from feedback:

Help Menu
Some people have a hard time discovering features or were not aware Firefox supported some of the features they wanted. To make it easier to learn more about Firefox, we added a simple Help menu which directs you to SUMO, our online support system.

Managing Home Panels
Not everyone loves the Firefox Homepage (I do!), or more specifically, they don’t like some of the panels. We added a simple way for people to control the panels shown in Firefox’s Homepage. You can change the default panel. You can even hide all the panels. Use Settings > Customize > Home to get there.

Home panels

Improve Top Sites
The Top Sites panel in the Homepage is used by many people. At the same time, other people find that the thumbnails can reveal a bit too much of their browsing to others. We recently added support for respecting sites that might not want to be snapshot into thumbnails. In those cases, the thumbnail is replaced with a favicon and a favicon-influenced background color. The Facebook and Twitter thumbnails show the effect below:


We also added the ability to remove thumbnails using the long-tap menu.

Manage Search Engines
People also like to be able to manage their search engines. They like to switch the default. They like to hide some of the built-in engines. They like to add new engines. We have a simple system for managing search engines. Use Settings > Customize > Search to get there.


Clear History
We have a lot of feedback from people who want to clear their browsing history quickly and easily. We are not sure if the Settings > Privacy > Clear private data method is too hard to find or too time consuming to use, but it’s apparent people need other methods. We added a quick access method at the bottom of the History panel in the Homepage.


We are also working on a Clear data on exit approach too.

Quickly Switch to a Newly Opened Tab
When you long-tap on a link in a webpage, you get a menu that allows you to Open in New Tab or Open in New Private Tab. Both of those open the new tab in the background. Feedback indicates the some people really want to switch to the new tab. We already show an Android toast to let you know the tab was opened. Now we add a button to the toast allowing you to quickly switch to the tab too.


Undo Closing a Tab
Closing tabs can be awkward for people. Sometimes the [x] is too easy to hit by mistake or swiping to close is unexpected. In any case, we added the ability to undo closing a tab. Again, we use a button toast.


Offer to Setup Sync from Tabs Tray
We feel that syncing your desktop and mobile browsing data makes browsing on mobile devices much easier. Figuring out how to setup the Sync feature in Firefox might not be obvious. We added a simple banner to the Homepage to let you know the feature exists. We also added a setup entry point in the Sync area of the Tabs Tray.


We’ll continue to make changes based on your feedback, so keep sending it to us. Thanks for using Firefox for Android!

Firefox for Android: Page Load Performance

One of the common types of feedback we get about Firefox for Android is that it’s slow. Other browsers get the same feedback and it’s an ongoing struggle. I mean, is anything ever really fast enough?

We tend to separate performance into three categories: Startup, Page Load and UX Responsiveness. Lately, we have been focusing on the Page Load performance. We separate further into Objective (real timing) and Subjective (perceived timing). If something “feels” slow it can be just as bad as something that is measurably slow. We have a few testing frameworks that help us track objective and subjective performance. We also use Java, JavaScript and C++ profiling to look for slow code.

To start, we have been focusing on anything that is not directly part of the Gecko networking stack. This means we are looking at all the code that executes while Gecko is loading a page. In general, we want to reduce this code as much as possible. Some of the things we turned up include:

  • Writing extra and/or redundant page information to the history database
  • Saving session restore information too frequently
  • Looking up Android system proxy information was slow
  • Drawing unused view backgrounds
  • Animating the pageload spinner consumed a lot of CPU cycles
  • Extra messaging between the Java UI and Gecko
  • Opportunities for predictive networking hints

Some of these were small improvements, while others, like the proxy lookups, were significant for “desktop” pages. I’d like to expand on two of the improvements:

Predictive Networking Hints
Gecko networking has a feature called Speculative Connections, where it’s possible for the networking system to start opening TCP connections and even begin the SSL handshake, when needed. We use this feature when we have a pretty good idea that a connection might be opened. We now use the feature in three cases:

  • When a touch occurs on a link in web content, we speculatively connect to the HREF. Technically, we need to wait before really opening the link to make sure you didn’t really start panning the page or start a double-tap.
  • When starting to type a URL in the titlebar, we load the current set of search engines. We speculative connect to the default search engine, since it’s somewhat probable that you will perform a search.
  • When typing into the titlebar, Firefox tries to use domain autocompletion and searches your history for recent URLs. Both the current auto-completed URL and the top-most search result are speculatively connected.

Animating the Page Load Spinner
Firefox for Android has used the animated spinner as a page load indicator for a long time. We use the Android animation framework to “spin” an image. Keeping the spinner moving smoothly is pretty important for perceived performance. A stuck spinner doesn’t look good. Profiling showed a lot of time was being taken to keep the animation moving, so we did a test and removed it. Our performance testing frameworks showed a variety of improvements in both objective and perceived tests.

We decided to move to a progressbar, but not a real progressbar widget. We wanted to control the rendering. We did not want the same animation rendering issues to happen again. We also use only a handful of “trigger” points, since listening to true network progress is also time consuming. The result is an objective page load improvement the ranges from ~5% on multi-core, faster devices to ~20% on single-core, slower devices.


The progressbar is currently allowed to “stall” during a page load, which can be disconcerting to some people. We will experiment with ways to improve this too.

Install Firefox for Android Nightly and let us know what you think of the page load improvements.