Fennec – Add-ons and Image Sizes

Building an add-on for Fennec (mobile Firefox) can be a bit tedious. We release desktop versions of Fennec that allow developers to test and play with their add-ons without the need for a mobile device. However, there are a few things that the desktop versions of Fennec do not expose: Performance characteristics of running on a mobile device; and the affect of small screens and high DPI.

I have blogged before about the potential performance issues and we have created some documents to help developers watch out for problems. I’ve blogged about mobile screen sizes before – about how you can use CSS to handle different sizes and orientations. This time I wanted to make the point about images sizes and the effect of DPI on fixed sized images.

On the desktop version of Firefox, 16px images are used for many of the UI elements. The trend in mobile device screens is big screens and high DPI. The DPI on desktop monitors has been below 100 for many years. Recently, we are finding monitors and laptops with higher DPIs. However, mobile devices can have displays with above 200 DPI, some even hit 300 DPI. Even the crappy iPhone display has 160 DPI. Using a 16px image on a 200 DPI (or greater) display will look tiny. It also has usability problems if the image is part of a touchable element. Fennec tries to keep touchable UI elements at ~ 6mm.

With the DPI issue in mind, add-on developers should really never use 16px images unless you are sure Fennec is running on a low DPI device. The Fennec UI uses 32px images for all favicons, list images and button images on high resolution screens. You can use the same CSS media queries I blogged about to control the images used in your UI:

/* high-res screens */
@media all and (min-device-width: 401px) {
  #myimage {
    list-style-image: url(chrome://myaddon/skin/images/cool-image-32.png);

/* low-res screens */
@media all and (max-device-width: 400px) {
  #myimage {
    list-style-image: url(chrome://myaddon/skin/images/cool-image-16.png);

You should not hard-code image URLs in XUL. This is considered bad practice:

<image src="chrome://myaddon/skin/images/cool-image-16.png"/>

Also note that the Fennec CSS is designed to stretch some element images to 32px. If you use 16px images for things like favicons or search providers, the result will be a pixelated mess.

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