Starting Points:

Mozilla Central / l10n: This contains the main mercurial (hg) repository for development of Gecko, XULRunner, and Firefox.
Mozilla 2.0 / l10n: This is the Mozilla 2.0 branch. (e.g. Firefox 4.0)
Mozilla 1.9.2 / l10n: This is the Mozilla 1.9.2 branch. (e.g. Firefox 3.6)
Mozilla 1.9.1 / l10n: This is the Mozilla 1.9.1 branch. (e.g. Firefox 3.5)
Comm. Central: This contains the mercurial (hg) repository for Thunderbird, SeaMonkey, and Calendar.
Comm. 1.9.2: This contains the 1.9.2 branch of the mercurial (hg) repository for Thunderbird, SeaMonkey, and Sunbird.
Comm. 1.9.1: This contains the 1.9.1 branch of the mercurial (hg) repository for Thunderbird, SeaMonkey, and Sunbird.
Mozilla CVS: This contains the entire current CVS repository. For Gecko, XULRunner, and Firefox, CVS trunk is no longer the trunk, and is instead used for Gecko 1.9 / Firefox 3 and the 1.9.0.* / 3.0.* security releases.
SeaMonkey / l10n: This the old code in CVS for Gecko, XULRunner, Firefox, Thunderbird, Calendar, Camino, and SeaMonkey. For Gecko, XULRunner, and Firefox. CVS trunk is only used for Gecko 1.9.0 / Firefox 3 and the 1.9.0.* / 3.0.* security releases.
Firefox 3: The source specific to the Firefox 3 web browser. Also available is the Firefox 2.0 Branch.
mobile-browser: This is the Fennec mobile web browser.
Camino: This is the source code of the Camino browser for Mac OS X.
Mozilla 1.8 / l10n: This is the Mozilla 1.8 branch. This module also hosts Aviary's 1.5 Branch (e.g. Firefox 2.0), for Mozilla toolkit derived projects.
Mozilla 1.8.0 / l10n: This is the Mozilla 1.8.0 branch. This is for security releases products released from Mozilla 1.8 Release (e.g. Firefox 1.5).
Aviary Branch / l10n: This module is Aviary's 1.0.1 Branch, Mozilla toolkit derived projects.
Classic: This is Mozilla Classic. It's a snapshot of the MozillaSource module from Oct 26, 1998 just before the change was made to xpfe. This is here for reference. No work is done on this branch.
NSPR: This module is NSPR, a cross platform library for operating system facilities including threads, I/O, timing and memory management.
Security: This module contains code related to Open Source PKI including NSS.
FUEL: This is FUEL, a JavaScript library designed to help developers build extensions using terminology and interfaces that are familiar to them.
JavaScript: This is Mozilla JavaScript. It includes SpiderMonkey, Rhino, Epimetheus, as well as debuggers and test suites.
Tamarin: This is Tamarin
Webtools / Webtools (CVS): These are the Mozilla Webtools. They include BZAPI, Despot, DXR, Litmus, MCS, MXR, Nightly, Bonsai, Tinderbox (2 and 3), Mozbot.
Build: Build/Release tools and configuration files
Bugzilla (3.2.x, 3.0.x, 2.22.x, 2.20.x): This is Bugzilla See docs.
Projects: This is the home for incomplete mozilla-central projects.
Services: All production code associated with the Weave project, including Firefox Sync, Firefox Home, and the server backends.
Incubator: This is the temporary home for mozilla.org projects. See the Incubator Repositories Policy.
Mozilla Labs Projects: This module contains code related to Mozilla Labs Projects Jetpack-SDK, Bespin, Raindrop, Weave, etc.
Mozilla Users: This is the home for mozilla.org developers' miscellaneous shared projects.
amo: This is the addons.mozilla.org website.
mozilla-org: This is the www.mozilla.org website.
spreadfirefox: This is the www.spreadfirefox.com website.

About Cross-References

This is a cross reference designed to display the Mozilla source code. The sources displayed are those that are currently checked in to the mainline of the mozilla.org CVS server, Mercurial Server, and Subversion Server; these pages are updated many times a day, so they should be pretty close to the latest‑and‑greatest.

It's possible to search through an entire source text; or to search for files whose name matches a pattern; or to search for the definitions of particular functions, variables, etc.

The individual files of the source code are formatted on the fly and presented with clickable identifiers. An identifier is a macro, typedef, struct, enum, union, function, function prototype or variable. Clicking on them shows you a summary of how and where they are used.

The free-text search command is implemented using Glimpse, so all the capabilities of Glimpse are available. Regular expression searches are especially useful.

Glimpse 3.6 is available for general use in the belief that its license is tolerable. This site is using the current version of glimpse under the open source project and nonprofits license.

(Don't use a web-crawler to try and download all of these pages; the CGIs will feed you several gigabytes worth of generated HTML!)

(That's beyond the nearly 30 gigabytes worth of content that is indexed here.)

The pages here are generated by the Mozilla MXR tool, which is a hacked variant of the original LXR which was written to display the source code of the Linux kernel (LXR stands for ``Linux Cross-Reference''). Check out the original LXR site for more information. That LXR is now maintained on sourceforge.net.

Thanks to Arne Georg Gleditsch and Per Kristian Gjermshus, the authors of the LXR tool, for writing it and making it available to the world; and thanks to Dawn Endico for doing almost all of the work to get LXR working with the Mozilla sources, and Josh Soref for the more recent updates to make it compatible with Subversion and Mercurial.

Issues with this installation of MXR can be filed in Bugzilla.