Developing extensions for Firefox for Android

You approach the coding of an extension for Firefox for Android in the same way as you would for a desktop extension; using a text editor or tool of your choice to write the code. However, when you want to test and debug your extension you need to follow a different process, this article walks you through that process.

Set up your computer and Android emulator or device

Complete some one-off setup tasks on your computer and Android device.

On your development computer:

• Install or update web-ext to version 4.1.0 or later.
• Use the Android Studio SDK Manager or the sdkmanager command-line tool to install the Android Platform Tools
• Make sure you have adb installed (Linux, Mac, Windows) and in your PATH.

On your device or Android emulator:

• Install Firefox for Android Nightly
• Enable Android USB debugging on the device.
• Attach your device to the development computer using a USB cable. When prompted, allow USB debugging for the connection.
• In the settings view for Firefox for Android Nightly, enable "Remote debugging via USB."

Then, on your development computer:

• Open a command shell.
• Run adb devices
  You should see output similar to:

  List of devices attached
  51800F220F01564 device

  Where the hex string is your device’s (or emulator’s) code. This means adb has found your device (or emulator).

Check for Firefox for Android compatibility

Lint with web-ext

Before running your extension on Firefox for Android, consider using web-ext lint. Lint performs a check to determine if any of the permissions, manifest keys, and web extension APIs you're using are incompatible with Firefox for Android. Lint relies on your extension's manifest.json file including strict_min_version values for the optional gecko and gecko_android sub-keys in browser_specific_settings. It then reports on the features that are not supported by the minimum version you have set.

In the lint report:

• incompatible permissions are identified by PERMISSION_FIREFOX_ANDROID_UNSUPPORTED_BY_MIN_VERSION,
• incompatible manifest keys are identified by KEY_FIREFOX_ANDROID_UNSUPPORTED_BY_MIN_VERSION, and
• incompatible APIs are identified by ANDROID_INCOMPATIBLE_API

similar to this:

When setting strict_min_version values in browser_specific_settings, unless you're targeting a specific version of Firefox, choose the most recent version of Firefox you expect your extension to be compatible with. Due to Android's different APIs and form factors (compared to desktop Firefox), set gecko_android after explicitly verifying compatibility. This sub-key enables a compatibility range that is distinct from Firefox for desktop.

"browser_specific_settings": {
  "gecko": {
    "strict_min_version": "102.0"
  },
  "gecko_android": {
    "strict_min_version": "113.0"
  }
}

Android API and UX checklist

Before making your extension available, test it on Android to ensure it works as expected. When you publish it on AMO, reviewers may reject it or request changes if it doesn't meet basic usability requirements.

• If you haven't used web-ext lint, check Browser Support for JavaScript APIs to verify that the APIs your extension uses are available on Firefox for Android.

• Make sure HTML pages have viewport meta tags to scale your extension's UI on mobile screens properly.

• Use responsive design patterns to ensure that your UI meets the expectations of Android users.

• Go through all of your extension's critical paths on an Android device to ensure they work as expected.

• Make sure the extension works well without an internet connection, as connectivity can be unreliable on mobile devices.

• Test your extension on a variety of device types and sizes. Android Studio's virtual devices can help.

Install and run your extension in Firefox for Android

In the unzipped directory of your extension, run web-ext run -t firefox-android and follow the instructions on screen to make sure you select the right device. Select org.mozilla.fenix as the apkname (or org.mozilla.firefox_beta for Firefox Beta for Android).

Here is an example of the command:

web-ext run -t firefox-android --adb-device XXX --firefox-apk org.mozilla.fenix

Debug your extension

You can debug your extension in the web developer tools and view any manifest.json validation messages using adb logcat. To make use of these features, first set up Firefox remote debugging over USB or via Wi-Fi.

Using web development tools to debug your extension

With your device connected over USB or Wi-Fi, open about:debugging and enable the device connection.

Your device is listed in the left-hand column, click Connect.

If prompted, allow the incoming connection on your Android device. Now start your extension on the Android device using web-ext run. You will need to have at least one tab opened in order for your extension to load. Click your device in the left-hand column and scroll down to find Processes in the list of active features in the browser.

Click Inspect next to Main Process. The web developer toolbox displays in Debugger.

For much of the debugging work, it's useful to be able to view Console with Inspector or Debugger. You do this using the split console, press esc to activate this mode.

Load a page in which your extension exercises. Now you can access any of the JavaScript in your extension.

In the Debugger you can set breakpoints, step through code, modify the extension's state, and do everything else you'd expect to be able to do in a debugger. Any messages logged by your code display in Console.

To inspect the popup's HTML and CSS, use Inspector. First, click the page select icon () to open the HTML document you want to inspect. You can review and modify the document's HTML and CSS in Inspector, as you would with any webpage.

For more details on using the web developer tools, see Firefox Developer Tools.

Viewing manifest validation messages using the console

In addition to the messages from your code, the console may also include messages about the validation of the extension's manifest.json files. These messages can be viewed using the adb logcat command. To avoid receiving other unrelated messages, you can pipe the output through grep, filtering by the extension's ID. For example:

/path/to/adb logcat | grep borderify@example.com

This will give output similar to this:

I/Gecko (30440): 1496056181889 addons.xpi WARN Addon with ID borderify@example.com already installed, older version will be disabled

If your add-on fails to run, check these messages as they may provide information explaining why.

Manifest V3 compatibility

Firefox for Android doesn't have full feature parity with the desktop version of Firefox. There are several known issues with Firefox for Android that can lead to a poor user experience. Therefore, it's recommended you use Manifest V2 for extensions targeting Firefox for Android.

Issue description	Tracking bug
Background service workers aren't supported on Firefox for Android. Instead, use event pages.	Bug 1573659
Firefox for Android doesn't visually indicate when an extension has a pending "host_permissions" grant request for the user.	Bug 1820867
Users can't edit an extension's host permission grants in Add-ons Manager on Firefox for Android.	Bug 1812125