How to contribute to Firefox

The goal of this doc is to have a place where all simple commands are listed from start to end.

This aims to be a simple tutorial for lazy copy and paste.

Each section in this tutorial links to more detailed documentation on the topic.

Clone the sources

We use mercurial, to clone the source:

$ hg clone

The clone should be around 30 minutes (depending on your connection) and the repository should be less than 5GB (~ 20GB after the build).

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Install dependencies

Firefox provides a mechanism to install all dependencies; in the source tree:

$ ./mach bootstrap

The default options are recommended. Select “Artifact Mode” if you are not planning to write C++ or Rust code.

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Windows dependencies

  1. You need 64-bit version of Windows 7 or later.
  2. Download and install Visual Studio.
  3. Finally download the MozillaBuild Package. Installation directory should be:
$ c:\mozilla-build\

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To build & run

Once all the dependencies have been installed, run:

$ ./mach build

which will check for dependencies and start the build. This will take a while; a few minutes to a few hours depending on your hardware.

To run it:

$ ./mach run

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To write a patch

Make the changes you need in the code base.


$ hg commit -m “Bug xxxx - the description of your change r?reviewer”

To find a reviewer, the easiest way is to do an “hg log” on the modified file and look who usually is reviewing the actual changes (ie not reformat, renaming of variables, etc).

To visualize your patch in the repository, run:

$ hg wip

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To test a change locally

To run the tests, use mach test with the path. However, it isn’t always easy to parse the results.

$ ./mach test dom/serviceworkers

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To test a change remotely

Running all the tests for Firefox takes a very long time and requires multiple operating systems with various configurations. To build Firefox and run its tests on continuous integration servers (CI), two commands are available:

$ ./mach try chooser

To select jobs running a fuzzy search:

$ ./mach try fuzzy

From Treeherder, it is also possible to attach new jobs. As every review has a try CI run associated, it makes this work easier. See Attaching new jobs from a review for more information.

Note that it requires level 1 permissions.

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To submit a patch

To submit a patch for review, we use a tool called moz-phab.

$ moz-phab

It will publish all the currently applied patches to Phabricator and inform the reviewer.

If you wrote several patches on top of each other:

$ moz-phab submit <first_revision>::<last_revision>

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To update a submitted patch

It is rare that a reviewer will accept the first version of patch. Moreover, as the code review bot might suggest some improvements, changes to your patch may be required.


$ hg commit --amend <the modified file>

If you wrote many changes, you can squash or edit commits with the command:

$ hg histedit

(similar to git rebase -i)

The submission is the same as a the initial patch.

Retrieve new changes from the repository

To pull changes from the repository, run:

$ hg update

If needed, to rebase a patch, run:

$ hg rebase -s <origin_revision> -d <destination_revision>

To push a change in the code base

Once the change has been accepted, ask the reviewer if they could land the change. They don’t have an easy way to know if a contributor has permission to land it or not.

If the reviewer does not land the patch after a few days, add the Check-in Needed Tags to the review (Edit Revision).

The landing procedure will automatically close the review and the bug.

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