Starting Points:

Mozilla Central / l10n: This contains the main mercurial (hg) repository for Gecko, XULRunner, and Firefox, the main line of development for releases after Gecko 1.9.1 / Firefox 3.5.
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 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 module is SeaMonkeyAll, the Mozilla browser project.
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.
Mozilla 1.0: This is the Mozilla 1.0 release.
Mozilla 1.0.x: This is the Mozilla 1.0 branch.
Mozilla 1.4.x: This is the Mozilla 1.4 branch.
Mozilla 1.7: This is the Mozilla 1.7 branch.
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.
M10: This is the source of the M10 release and is provided here for use by the design patterns contest.
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.
Labs: This module contains code related to Mozilla Labs, including Weave.
Grendel: This module is Grendel, a mail reader written in java.
Electrical Fire: This is /mozilla/ef, a multi-platform Just‑In‑Time Java compiler.
GCC 4.0.2: This contains the entire gcc source for 4.0.2. In particular, it has G++.
Tamarin ( central, tracing ): Tamarin is the replacement for SpiderMonkey. It was originally donated by Adobe and is the core for ActionScript (Flash 9). Tracing is an experimental version (?).
JavaScript: These are Mozilla's home grown JavaScript engines. They includes SpiderMonkey, Rhino, Epimetheus, as well as debuggers and test suites.
Webtools: These are the Mozilla Webtools. They include Bugzilla, Bonsai, Tinderbox (2 and 3), MXR, Mozbot.
Build: Build/Release tools and configuration files
Bugzilla ( 3.2.x, 3.0.x, 2.22.x, 2.20.x, 2.18.x, 2.16.x ): This is Bugzilla See docs.
amo: This is the addons.mozilla.org website.
mozilla-org: This is the mozilla.org website.
mozillafoundation-org: This is the mozillafoundation.org website.
mozilla-com: This is the mozilla.com website.
spreadfirefox: This is the spreadfirefox.com website.
devmo: This is the developer.mozilla.org website.
devmowiki: This is the developer.mozilla.org/wiki website.
chrome: This is Google Chrome.
html5: This is HTML5.

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.