There are lots of ways to contribute to the Mozilla project: coding, testing, improving the build process and tools, or contributing to the documentation. This guide provides information that will not only help you get started as a Mozilla contributor, but that you'll find handy to refer to even as an old hand.

Documentation topics

Getting Started: A step-by-step beginner's guide to getting involved with Mozilla.

Working with Mozilla Source Code: A code overview, how to get the code, and the coding style guide.
Build Instructions: How to build Firefox, Thunderbird, SeaMonkey, or other Mozilla applications.
Development process overview: An overview of the entire Mozilla development process.
Managing multiple profiles: When working with prerelease versions of Firefox, it's often helpful to have multiple Firefox profiles, such as one for each channel, or for different kinds of testing.
Automated Testing: How to run Mozilla's automated tests, and how to write new tests.
How to submit a patch: After getting your patch written, you need to get it checked into the tree. This article explains the review process and how to get your patch approved.
Getting documentation updated: How to ensure that documentation is kept up to date as you develop.
Mozilla modules and module ownership: This article provides information about Mozilla's modules, what the role of a module owner is, and how module owners are selected.
Code snippets: Useful code samples for a wide variety of things you might need to figure out how to do.
Mozilla development strategies: Tips for how to make the most of your time working on the Mozilla project.
Debugging: Find helpful tips and guides for debugging Mozilla code.
Performance: Performance guides and utilities to help you make your code perform well (and to play nicely with others).
The Mozilla platform: Information about the workings of the Mozilla platform.
Adding APIs to the navigator object {{ gecko_minversion_inline("9.0") }}: How to augment the {{ domxref("window.navigator") }} object with additional APIs.
Interface Compatibility: Guidelines for modifying scriptable and binary APIs in Mozilla.
Customizing Firefox: Information about creating customized versions of Firefox.
Virtual ARM Linux environment: How to set up an ARM emulator running Linux for testing ARM-specific, but not necessarily platform-specific, code. Useful for mobile developers.
Obsolete Build Caveats and Tips: A place to put build tips which are no longer relevant to building the latest version of the code from main but are relevant when building old codebases.

Tools

Bugzilla: The Bugzilla database used to track issues for Mozilla projects.
MXR: Browse and search the Mozilla source code repository on the Web.
DXR: Next generation of searching Mozilla's source code. In active development.
Bonsai: The Bonsai tool lets you find out who changed what file in the repository, and when they did it.
Mercurial: The distributed version-control system used to manage Mozilla's source code.
TBPL (Tinderbox Push Log): Tinderbox Push Log shows the status of the tree (whether or not it currently builds successfully). Check this before checking in and out, to be sure you're working with a working tree.
Crash tracking: Information about the Socorro crash reporting system.
Performance tracking: Datazilla and Graphs: See performance information for Mozilla projects.
Callgraph: A tool to help perform static analysis of the Mozilla code by generating callgraphs automatically.
Developer forums: A topic-specific list of discussion forums where you can talk about Mozilla development issues.
Mozilla Platform Development Cheat Sheet: Brian Bondy's list of frequently referenced information for platform developers.
Firefox development video tutorials: Brian Bondy's video tutorials on Firefox development.