Contributor Guide

Table of contents

GeckoView Contributor Quick Start Guide

This is a guide for developers who want to contribute to the GeckoView project. If you want to get started using GeckoView in your app then you should refer to the wiki.

Get set up with Mozilla Central

The GeckoView codebase is part of the main Firefox tree and can be found in mozilla-central. You will need to get set up as a contributor to Firefox in order to contribute to GeckoView. To get set up with mozilla-central, follow the Quick Start Guide for Git Users, or the Contributing to the Mozilla code base guide and Firefox Contributors’ Quick Reference for Mercurial users.

Once you have a copy of mozilla-central, you will need to build GeckoView.

Bootstrap Gecko

Bootstrap configures everything for GeckoView and Fennec (Firefox for Android) development.

  • Ensure you have mozilla-central checked out. If this is the first time you are doing this, it may take some time.

git checkout central/default

If you are on a mac, you will need to have the Xcode build tools installed. You can do this by either installing Xcode or installing only the tools from the command line by running xcode-select --install and following the on screen instructions.

You will need to bootstrap for GeckoView/Firefox for Android. The easiest way is to run the following command:

./mach --no-interactive bootstrap --application-choice="GeckoView/Firefox for Android"


  • The --no-interactive argument will make bootstrap run start to finish without requiring any input from you. It will automatically accept any license agreements.

  • The --application-choice="GeckoView/Firefox for Android" argument is needed when using --no-interactive so that “bootstrapping” is done for the correct application (instead of the default).

If you want to make all the selections yourself and/or read through the license agreements, you can simply run:

./mach bootstrap

Select 4. GeckoView/Firefox for Android when prompted and respond to any subsequent prompts as they appear.

Once ./mach bootstrap is complete, it will automatically write the configuration into a new mozconfig file. If you already have a mozconfig, mach will instead output new configuration that you should append to your existing file.

Build from the command line

In order to pick up the configuration changes we just made we need to build from the command line. This will update generated sources, compile native code, and produce GeckoView AARs and example and test APKs.

./mach build

Build Using Android Studio

  • Install Android Studio.

  • Choose File->Open from the toolbar

  • Navigate to the root of your mozilla-central source directory and click “Open”

  • Click yes if it asks if you want to use the gradle wrapper.

    • If the gradle sync does not automatically start, select File > Sync Project with Gradle Files.

  • Wait for the project to index and gradle to sync. Once synced, the workspace will reconfigure to display the different projects.

    • annotations contains custom Java annotations used inside GeckoView

    • app contains geckoview build settings and omnijar. omnijar contains the parts of Gecko and GeckoView that are not written in Java or Kotlin

    • geckoview is the GeckoView project. Here is all the Java files related to GeckoView

    • geckoview_example is an example browser built using GeckoView.

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Now you’re set up and ready to go.

Important: at this time, building from Android Studio or directly from Gradle does not (re-)compile native code, including C++ and Rust. This means you will need to run mach build yourself to pick up changes to native code. Bug 1509539 tracks making Android Studio and Gradle do this automatically.

If you want set up code formatting for Kotlin, please reference IntelliJ IDEA configuration.

Custom mozconfig with Android Studio

Out of the box, Android Studio will use the default mozconfig file, normally located at mozconfig in the root directory of your mozilla-central checkout.

To make Android Studio use a mozconfig in a custom location, you can add the following to your


Note that, when running mach from the command line, this value will be ignored, and the mozconfig from the mach environment will be used instead.

To override the mozconfig used by mach, you can use the MOZCONFIG environment variable, for example:

MOZCONFIG=debug.mozconfig ./mach build

Performing a bug fix

One you have got GeckoView building and running, you will want to start contributing. There is a general guide to Performing a Bug Fix for Git Developers for you to follow. To contribute to GeckoView specifically, you will need the following additional information.

Running tests and linter locally

To ensure that your patch does not break existing functionality in GeckoView, you can run the junit test suite with the following command

./mach geckoview-junit

This command also allows you to run individual tests or test classes, e.g.

./mach geckoview-junit org.mozilla.geckoview.test.NavigationDelegateTest
./mach geckoview-junit org.mozilla.geckoview.test.NavigationDelegateTest#loadUnknownHost

If your patch makes a GeckoView JavaScript module, you should run ESLint as well:

./mach lint -l eslint mobile/android/modules/geckoview/

To see information on other options, simply run ./mach geckoview-junit --help; of particular note for dealing with intermittent test failures are --repeat N and --run-until-failure, both of which do exactly what you’d expect.

Updating the changelog and API documentation

If the patch that you want to submit changes the public API for GeckoView, you must ensure that the API documentation is kept up to date. To check whether your patch has altered the API, run the following command.

./mach lint --linter android-api-lint

The output of this command will inform you if any changes you have made break the existing API. Review the changes and follow the instructions it provides.

If the linter asks you to update the changelog, please ensure that you follow the correct format for changelog entries. Under the heading for the next release version, add a new entry for the changes that you are making to the API, along with links to any relevant files, and bug number e.g.

- Added [`GeckoRuntimeSettings.Builder#aboutConfigEnabled`][71.12] to control whether or
  not `about:config` should be available.
  ([bug 1540065]({{bugzilla}}1540065))

[71.12]: {{javadoc_uri}}/GeckoRuntimeSettings.Builder.html#aboutConfigEnabled(boolean)

Submitting to the try server

It is advisable to run your tests before submitting your patch. You can do this using Mozilla’s try server. To submit a GeckoView patch to try before submitting it for review, type:

./mach try auto

This will automatically select tests to run from our suite. If your patch passes on try you can be (fairly) confident that it will land successfully after review.

Tagging a reviewer

When submitting a patch to Phabricator, if you know who you want to review your patch, put their Phabricator handle against the reviewers field.

If you don’t know who to tag for a review in the Phabricator submission message, leave the field blank and, after submission, follow the link to the patch in Phabricator and scroll to the bottom of the screen until you see the comment box.

  • Select the Add Action drop down and pick the Change Reviewers option.

  • In the presented box, add geckoview-reviewers. Selecting this group as the reviewer will notify all the members of the GeckoView team there is a patch to review.

  • Click Submit to submit the reviewer change request.

Include GeckoView as a dependency

If you want to include a development version of GeckoView as a dependency inside another app, you must link to a local copy. There are several ways to achieve this, but the preferred way is to use Gradle’s dependency substitution mechanism, for which there is first-class support in mozilla-central and a pattern throughout Mozilla’s GeckoView-consuming ecosystem.

The good news is that mach build produces everything you need, so that after the configuration below, you should find that the following commands rebuild your local GeckoView and then consume your local version in the downstream project.

cd /path/to/mozilla-central && ./mach build
cd /path/to/project && ./gradlew assembleDebug

Be sure that your ``mozconfig`` specifies the correct ``–target`` argument for your target device. Many projects use “ABI splitting” to include only the target device’s native code libraries in APKs deployed to the device. On x86-64 and aarch64 devices, this can result in GeckoView failing to find any libraries, because valid x86 and ARM libraries were not included in a deployed APK. Avoid this by setting --target to the exact ABI that your device supports.

Dependency substiting your local GeckoView into a Mozilla project

Most GeckoView-consuming projects produced by Mozilla support dependency substitution via These projects include:

Simply edit (or create) the file in the project root and include a line like:


The default object directory – the one that a plain mach build discovers – will be used. You can optionally specify a particular object directory with an additional line like:


With these lines, the GeckoView-consuming project should use the GeckoView AAR produced by mach build in your local mozilla-central.

Remember to remove the lines in ```` when you want to return to using the published GeckoView builds!

Dependency substituting your local GeckoView into a non-Mozilla project

In projects that don’t have first-class support for dependency substitution already, you can do the substitution yourself. See the documentation in substitue-local-geckoview.gradle, but roughly: in each Gradle project that consumes GeckoView, i.e., in each build.gradle with a dependencies { ... 'org.mozilla.geckoview:geckoview-...' } block, include lines like:

ext.topsrcdir = "/path/to/mozilla-central"
ext.topobjdir = "/path/to/object-directory" // Optional.
apply from: "${topsrcdir}/substitute-local-geckoview.gradle"

Remember to remove the lines from all ``build.gradle`` files when you want to return to using the published GeckoView builds!

Next Steps