Firefox on Mobile: A/B Testing and Staged Rollouts


We have decided to start running A/B Testing in Firefox for Android. These experiments are intended to optimize specific outcomes, as well as, inform our long-term design decisions. We want to create the best Firefox experience we can, and these experiments will help.

The system will also allow us to throttle the release of features, called staged rollout or feature toggles, so we can monitor new features in a controlled manner across a large user base and a fragmented device ecosystem. If we need to rollback a feature for some reason, we’d have the ability to do that, quickly without needing people to update software.

Technical details:

  • Mozilla Switchboard is used to control experiment segmenting and staged rollout.
  • UI Telemetry is used to collect metrics about an experiment.
  • Unified Telemetry is used to track active experiments so we can correlate to application usage.

What is Mozilla Switchboard?

Mozilla Switchboard is based on Switchboard, an open source SDK for doing A/B testing and staged rollouts from the folks at KeepSafe. It connects to a server component, which maintains a list of active experiments.

The SDK does create a UUID, which is stored on the device. The UUID is sent to the server, which uses it to “bucket” the client, but the UUID is never stored on the server. In fact, the server does not store any data. The server we are using was ported to Node from PHP and is being hosted by Mozilla.

We decided to start using Switchboard because it’s simple, open source, has client code for Android and iOS, saves no data on the server and can be hosted by Mozilla.

Planning Experiments

The Mobile Product and UX teams are the primary drivers for creating experiments, but as is common on the Mobile team, ideas can come from anywhere. We have been working with the Mozilla Growth team, getting a better understanding of how to design the experiments and analyze the metrics. UX researchers also have input into the experiments.

Once Product and UX complete the experiment design, Development would land code in Firefox to implement the desired variations of the experiment. Development would also land code in the Switchboard server to control the configuration of the experiment: On what channels is it active? How are the variations distributed across the user population?

Since we use Telemetry to collect metrics on the experiments, the Beta channel is likely our best time period to run experiments. Telemetry is on by default on Nightly, Aurora and Beta; and Beta is the largest user base of those three channels.

Once we decide which variation of the experiment is the “winner”, we’ll change the Switchboard server configuration for the experiment so that 100% of the user base will flow through the winning variation.

Yes, a small percentage of the Release channel has Telemetry enabled, but it might be too small to be useful for experimentation. Time will tell.

What’s Happening Now?

We are trying to be very transparent about active experiments and staged rollouts. We have a few active experiments right now.

  • Onboarding A/B experiment with several variants.
  • Easy entry points for accessing History and Bookmarks on the main menu.
  • Experimenting with the awesomescreen behavior when displaying search results page.

You can always look at the Mozilla Switchboard configuration to see what’s happening. Over time, we’ll be adding support to Firefox for iOS as well.