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Firefox Preview

Task Status codecov

Firefox Preview (internal code name: “Fenix”) is an all-new browser for Android, based on GeckoView and Mozilla Android Components.

** Note: The team is currently experiencing heavy triage and review load, so when triaging issues, we will mainly be looking to identify S1 (high severity) issues. See our triage process here. Please be patient if you don’t hear back from us immediately on your issue! **

I want to open a Pull Request!

We encourage you to participate in this open source project. We love Pull Requests, Bug Reports, ideas, (security) code reviews or any other kind of positive contribution.

Since we are a small team, however, we do not have the bandwidth to review unsolicited PRs. Please follow our Pull Request guidelines, or we may close the PR.

To make it easier to review, we have these PR requirements:

  • Every PR must have exactly one issue associated with it.
  • Write a clear explanation of what the code is doing when opening the pull request, and optionally add comments to the PR.
  • Make sure there are tests - or ask for help on how the code should be tested in the Issue!
  • Keep PRs small and to the point. For extra code-health changes, either file a separate issue, or make it a separate PR that can be easily reviewed.
  • Use micro-commits. This makes it easier and faster to review.
  • Add a screenshot for UX changes (this is part of the PR checklist)

As a small team, we have to prioritize our work, and reviewing PRs takes time. We receive lots of PRs every day, so if you can keep your PRs small, it helps our small team review and merge code faster, minimizing stale code.

Keep in mind that the team is very overloaded, so PRs sometimes wait for a very long time. However this is not for lack of interest, but because we find ourselves in a constant need to prioritize certain issues/PRs over others. If you think your issue/PR is very important, try to popularize it by getting other users to comment and share their point of view.

I want to file an issue!

Great! We encourage you to participate in this open source project. We love Pull Requests, Bug Reports, ideas, (security) code reviews or any other kind of positive contribution.

To make it easier to triage, we have these issue requirements:

  • Please do your best to search for duplicate issues before filing a new issue so we can keep our issue board clean.
  • Every issue should have exactly one bug/feature request described in it. Please do not file meta feedback list tickets as it is difficult to parse them and address their individual points.
  • Feature Requests are better when they’re open-ended instead of demanding a specific solution -ie “I want an easier way to do X” instead of “add Y”
  • Issues are not the place to go off topic or debate. If you have questions, please join the channel.
  • Please always remember our Community Participation Guidelines
  • Please do not tag specific team members to try to get your issue looked at faster. We have a triage process that will tag and label issues correctly in due time. If you think an issue is very severe, you can ask about it in Matrix.

Please keep in mind that even though a feature you have in mind may seem like a small ask, as a small team, we have to prioritize our planned work and every new feature adds complexity and maintenance and may take up design, research, marketing, product, and engineering time. We appreciate everyone’s passion but we will not be able to incorporate every feature request or even fix every bug. That being said, just because we haven’t replied, doesn’t mean we don’t care about the issue, please be patient with our response times as we’re very busy.

Getting Involved

Before you attempt to make a contribution please read the Community Participation Guidelines.

Beginners! - Watch out for Issues with the “Good First Issue” label. These are easy bugs that have been left for first timers to have a go, get involved and make a positive contribution to the project!

Build Instructions

Note: Both Android SDK and NDK are required.

  1. Clone or Download the repository:
  git clone
  1. Import the project into Android Studio or build on the command line:
  ./gradlew clean app:assembleGeckoBetaDebug

Use app:assembleGeckoNightlyDebug to build with the Gecko Nightly version instead. If this errors out, make sure that you have an ANDROID_SDK_ROOT environment variable pointing to the right path.

  1. Make sure to select the correct build variant in Android Studio. See the next section.

Guide to Build Variants

We have a lot of build variants. Each variant is composed of two flavors. One flavor is the version of Gecko to use and the other describes which app id and settings to use. Here is a description of what each means:

  • geckoBeta (recommended) uses the Beta variant of the Gecko rendering engine, which corresponds to the next version of Gecko which will go to production
  • geckoNightly uses the Nightly variant of the Gecko rendering engine, which is the version which will arrive after beta and is less stable

  • debug uses debug symbols and debug signing, adds tools like LeakCanary for troubleshooting, and does not strip unused or wasteful code
  • fenixNightly is a release build with nightly signing which uses the org.mozilla.fenix.nightly app id for nightly releases to Google Play
  • fenixBeta is a release build with beta signing which uses the org.mozilla.fenix.beta app id for beta releases to Google Play
  • fenixProduction is a release build with release signing which uses the org.mozilla.fenix app id for production releases to Google Play
  • fennecProduction is an experimental build with release signing which uses the org.mozilla.firefox app id and supports upgrading the older Firefox. WARNING Pre-production versions of this may result in data loss.
  • forPerformanceTest: see “Performance Build Variants” below.

Performance Build Variants

For accurate performance measurements, read this section!

If you want to analyze performance during local development (note: there is a non-trivial performance impact - see caveats):

  • Recommendation: use a forPerformanceTest variant with local Leanplum, Adjust, & Sentry API tokens: contact the front-end perf group for access to them
  • Rationale: forPerformanceTest is a release variant with debuggable set to true. There are numerous performance-impacting differences between debug and release variants so we need a release variant. To profile, we also need debuggable, which is disabled on other release variants. If API tokens are not provided, the SDKs may change their behavior in non-trivial ways.
  • Caveats:
    • debuggable has a non-trivial & variable impact on performance but is needed to take profiles.
    • Random experiment opt-in & feature flags may impact performance (see perf-frontend-issues#45 for mitigation).
    • This is slower to build than debug builds because it does additional tasks (e.g. minification) similar to other release builds

Nightly forPerformanceTest variants with API tokens already added are also available from Taskcluster.

If you want to run performance tests/benchmarks in automation or locally:

  • Recommendation: production builds. If debuggable is required, use recommendation above but note the caveat above. If your needs are not met, please contact the front-end perf group to identify a new solution.
  • Rationale: like the rationale above, we need release variants so the choice is based on the debuggable flag.

For additional context on these recommendations, see the perf build variant analysis.

Before you can install any release variants including forPerformanceTest, you will need to sign them: see Automatically signing release builds for details.

Pre-push hooks

To reduce review turn-around time, we’d like all pushes to run tests locally. We’d recommend you use our provided pre-push hook in config/ Using this hook will guarantee your hook gets updated as the repository changes. This hook tries to run as much as possible without taking too much time.

To add it on Mac/Linux, run this command from the project root:

ln -s ../../config/ .git/hooks/pre-push

or for Windows run this command using the Command Prompt with administrative privileges:

mklink .git\hooks\pre-push ..\..\config\

or using PowerShell:

New-Item -ItemType SymbolicLink -Path .git\hooks\pre-push -Value (Resolve-Path config\

To push without running the pre-push hook (e.g. doc updates):

git push <remote> --no-verify

Note: If while pushing you encounter this error “Could not initialize class org.codehaus.groovy.runtime.InvokerHelper” and are currently on Java14 then downgrading your Java version to Java13 or lower can resolve the issue

Steps to downgrade Java Version on Mac with Brew:

  1. Install Homebrew (
  2. run brew update
  3. To uninstall your current java version, run sudo rm -fr /Library/Java/JavaVirtualMachines/<jdk-version>
  4. run brew tap homebrew/cask-versions
  5. run brew search java
  6. If you see java11, then run brew install java11
  7. Verify java-version by running java -version helpers

You can speed up local development by setting a few helper flags available in Some flags will make it easy to work across multiple layers of the dependency stack - specifically, with android-components, geckoview or application-services.

Automatically sign release builds

To sign your release builds with your debug key automatically, add the following to <proj-root>/


With this line, release build variants will automatically be signed with your debug key (like debug builds), allowing them to be built and installed directly through Android Studio or the command line.

This is helpful when you’re building release variants frequently, for example to test feature flags and or do performance analyses.

Auto-publication workflow for android-components and application-services

If you’re making changes to these projects and want to test them in Fenix, auto-publication workflow is the fastest, most reliable way to do that.

In, specify a relative path to your local android-components and/or application-services checkouts. E.g.:

  • autoPublish.application-services.dir=../application-services

Once these flags are set, your Fenix builds will include any local modifications present in these projects.

See a demo of auto-publication workflow in action.


Specify a relative path to your local mozilla-central checkout via dependencySubstitutions.geckoviewTopsrcdir, and optional a path to m-c object directory via dependencySubstitutions.geckoviewTopobjdir.

If these are configured, local builds of GeckoView will be used instead of what’s configured in Dependencies.kt. For more details, see


This Source Code Form is subject to the terms of the Mozilla Public
License, v. 2.0. If a copy of the MPL was not distributed with this
file, You can obtain one at