Contributing to the Mozilla code base

This page should guide you through the initial steps of contributing to Mozilla. Welcome, we're delighted to see you! :)

Need help?

The Mozilla community prides itself on being an open, accessible, and friendly community for new participants. If you have any difficulties getting involved or finding answers to your questions, please bring those questions to the #introduction chat room on, where we can help you get started.

We know even before you start contributing that getting set up to work on Firefox and finding a bug that's a good fit for your skills can be a challenge. We're always looking for ways to improve this process: making Mozilla more open, accessible, and easier to participate with. If you're having any trouble following this documentation, or hit a barrier you can't get around, please contact Mike Hoye at We're determined to solve hurdles for new contributors.

What skills do I need?

Mozilla is a large project and we are thrilled to have contributors with very diverse skills:

  • If you know C++, you can contribute to the core layers of Firefox and other Mozilla products
  • If you know JavaScript or HTML/CSS, you can contribute to the front-end of Firefox
  • If you know Java, you can contribute to Firefox on Android, Firefox Focus for Android and Firefox for Fire TV.
  • If you know Kotlin, you can contribute to Firefox Preview (code name: "Fenix").
  • If you know Swift, you can contribute to Firefox for iOS and Firefox Focus for iOS
  • If you know Python, you can contribute to our web services, including Firefox Sync, or Firefox Accounts
  • If you know Make, shell, Perl, or Python, you can contribute to our build systems, release engineering, and automation
  • If you know C, you can contribute to NSS, Opus, and Daala
  • If you know Rust, you can contribute to the Rust programming language; Servo, the web browser engine designed for parallelism and safety; or Quantum, the project to bring large pieces of Servo to Gecko
  • There are even many ways to contribute to the Mozilla mission without programming. If getting involved in design, support, translation, testing, or other types of contributions sparks your interest please see the Volunteer Opportunities wiki or the Activate community site.

Perhaps you do not know programming yet, but you want to start learning? There are plenty of resources available on the MDN Web Docs!

Step 1: Build Firefox for Desktop or Mobile

If you'd like to contribute to Firefox, simple instructions to build desktop or mobile Firefox are here. Getting set up may take some time as there are some big downloads involved, so you may want to move on to the next steps while it builds. Additional build instructions can be found here.

Mozilla's other products, including the community-supported Thunderbird builds, can be found with a quick search. Often you won't need to build anything to start making a contribution.

Step 2: Find something to work on

Bugs listed as 'Assigned' are not usually a good place to start, unless you're sure you have something worthy to contribute. Someone else is already working on it!
Even with no assignee, it is polite to check if someone has recently commented that they're looking at fixing the issue.

Once you have found something to work on, go ahead and comment! Let the bug submitter, reviewer, and component owner know that you'd like to work on the bug. You might receive some extra information, perhaps also made the assignee.

Fix your pet peeve

If there's something you'd like to fix about Firefox, Thunderbird, or your other favorite Mozilla application, this can be a great place to start. There are a number of ways to do this:

Find a bug we've identified as a good fit for new contributors.

With more than a million bugs filed in Bugzilla, it can be hard to know where to start, so we've created these bug categories to make getting involved a little easier:

  • Codetribute - our site for finding bugs that are mentored, some are good first bugs, some are slightly harder. Your mentor will help guide you with the bug fix and through the submission and landing process.
  • Good First Bugs - are the best way to take your first steps into the Mozilla ecosystem. They're all about small changes, sometimes as little as a few lines, but they're a great way to learn about setting up your development environment, navigating Bugzilla, and making contributions to the Mozilla codebase.
  • Follow @StartMozilla on Twitter - we link up Good First Bugs for new contributors across Mozilla every day.
  • Visit - we list Firefox Developer Tools bugs for new contributors.
  • Student Projects - are larger projects, such as might be suitable for a university student for credit. Of course, if you are not a student, feel free to fix one of these bugs. We maintain two lists: one for projects based on the existing codebase, and one for implementing new applications.

Use a bug searching tool

There are several tools developed by members of the Mozilla community. These help
new contributors find bugs to work on, without having to use Bugzilla's more perplexing search component.

Step 3: Fix the bug

We leave this in your hands. Here are some further resources to help:

Step 4: Get your code reviewed

Once you fix the bug, you can advance to having your code reviewed.

Mozilla uses Phabricator for code review.

Who is the right person to ask for a review?

  • If you have a mentored bug: ask your mentor. They will help, or can easily find out. It might be them!
  • Run hg blame on the file and look for the people who have touched the functions you're working on. They too are good candidates.
    Running hg log and looking for regular reviewers might be a solution too.
  • The bug itself may contain a clear indication of the best person to ask for a review
  • Are there related bugs on similar topics? The reviewer in those bugs might be another good choice
  • We have an out of date list of modules, which lists peers and owners for the module. Some of these will be good reviewers. In a worst case scenario, set the module owner as the reviewer, asking them in the comments to pick someone more suitable

Please select only one reviewer.

Step 4b: Follow it up

Once you've asked for a review, a reviewer will often respond within a day or two, reviewing the patch, or saying when they will be able to review it, perhaps due to a backlog. If you don't hear back within this time, naturally reach out to them: add a comment to the bug saying 'review ping?', check the "Need more information from" box, and add the reviewer's name. If they don't respond within a day or two, you can ask for help on IRC in the #introduction, or #developers channels, or contact Mike Hoye directly.

Don't hesitate to contact your mentor as well if this isn't moving.

Step 5: Respond to the review

For most new contributors, and even for long-time Mozillians, the first review of your patch will be "Requested Changes" (or an "r-" in Bugzilla). This does not mean you've done bad work. There is more work to do before the code can be merged into the tree. Your patch may need some changes - perhaps minor, perhaps major - and your reviewer will give you some guidance on what needs to be done next.

This is an important process, so don't be discouraged! With our long-lived codebase, and hundreds of millions of users, the care and attention helping contributors bring good patches is the cornerstone of the Mozilla project. Make any changes your reviewer seeks; if you're unsure how, be sure to ask! Attach the new patch to the bug again, and ask for a further review from the same reviewer. If they accept your changes, this means your patch can be landed into the tree!

Step 6: Getting code into the tree

Once your patch has been accepted, it is ready to go. Before it can be merged into the tree, your patch will need to complete a successful run through our try server, making sure there are no unexpected regressions. If you don't have try server access already, your mentor, or the person who reviewed your patch, will be able to help.

Once you have a green try server run, mark that your patch is ready to commit, by adding the checkin-needed keyword, to the "keywords" field at the top of the bug. A friendly Mozillian, with commit access, will be along shortly to push your patch to the repository, and update the bug as required. If your patch passes all Mozilla's automated testing, it will soon be merged into the main branch, and become a part of the Nightly build.

Step 7: Repeat

Thank you! You've fixed your very first bug, and the Open Web is stronger for it. But don't stop now.

Go back to step 3, as there is plenty more to do. Your mentor might suggest a new bug for you to work on, or find one that interests you.  Now that you've got your first bug fixed you should request level 1 access to the repository to push to the try server and get automated feedback about your changes on multiple platforms. After fixing a nontrivial number of bugs you should request level 3 access so you can land your own code after it has been reviewed.

More information

We're in the process of improving information on this page for newcomers to the project. We'll be integrating some information from these pages soon, but until then you may find them interesting in their current form: