Starting Points:

Mozilla Central / l10n
This contains the main mercurial (hg) repository for development of Gecko, XULRunner, and Firefox.
Mozilla Aurora / l10n
This is the Mozilla Aurora branch.
Mozilla Beta / l10n
This is the Mozilla Beta branch. (e.g. Firefox ## Beta)
Mozilla Release / l10n
This is the Mozilla Release branch. (e.g. Firefox ##)
Mozilla ESR24
ESR24 version of Firefox
Mozilla ESR31
ESR31 version of Firefox
Mozilla ESR38
ESR38 version of Firefox
Mozilla ESR45
ESR45 version of Firefox
Mozilla B2G28 v1_3
This is the Mozilla FirefoxOS 28 v1_3 branch. (for Gecko code)
Mozilla B2G28 v1_3t
This is the Mozilla FirefoxOS 28 v1_3t branch. (for Gecko code)
Mozilla B2G30 v1_4
This is the Mozilla FirefoxOS 30 v1_4 branch. (for Gecko code)
Mozilla B2G32 v2_0
This is the Mozilla FirefoxOS 32 v2_0 branch. (for Gecko code)
Mozilla B2G34 v2_1
This is the Mozilla FirefoxOS 34 v2_1 branch. (for Gecko code)
Mozilla B2G37 v2_2
This is the Mozilla FirefoxOS 37 v2_2 branch. (for Gecko code)
B2G - Gaia
Pulled from: https://github.com/mozilla-b2g/gaia
Gaia-l10n (v1_3, v1_2, v1_1, v1_0_1)
Gaia localization.
Comm. Central
This contains the mercurial (hg) repository for Thunderbird, SeaMonkey, and Calendar.
Comm. Aurora
Thunderbird and SeaMonkey aurora channel.
Comm. Beta
Thunderbird and SeaMonkey beta channel.
Comm. Release
Released versions of Thunderbird, Seamonkey, etc.
Comm. ESR10
ESR10 versions of Thunderbird, Seamonkey, etc.
Comm. ESR17
ESR17 versions of Thunderbird, Seamonkey, etc.
Comm. ESR24
ESR24 versions of Thunderbird, Seamonkey, etc.
Comm. ESR31
ESR31 versions of Thunderbird, Seamonkey, etc.
Comm. ESR38
ESR38 versions of Thunderbird, Seamonkey, etc.
Comm. ESR45
ESR45 versions of Thunderbird, Seamonkey, etc.
NSPR
This module is NSPR, a cross platform library for operating system facilities including threads, I/O, timing and memory management.
NSS
This module is NSS (Network Security Services), a set of libraries designed to support cross-platform development of security-enabled client and server applications.
JSS
This module is JSS (Network Security Services for Java), a Java interface to NSS.
python-nss
This module is python-nss, a Python binding for NSS.
NSPR CVS
This is the NSPR section of the CVS repository. The CVS repository is now read-only after the NSPR project migrated to Mercurial. See "NSPR" above.
Security CVS
This is the section of the CVS repository that contains NSPR, NSS, JSS, and python-nss. The CVS repository is now read-only after the NSS project migrated to Mercurial. See "NSS" above.'
Chromium
This is the latest "last known good revision" of Chromium trunk.
SeaMonkey / l10n
This the old code in CVS for Gecko, XULRunner, Firefox, Thunderbird, Calendar, Camino, and SeaMonkey. CVS trunk is only used for Gecko 1.9.0 / Firefox 3 and the 1.9.0.* / 3.0.* security releases.
Camino
This is the source code of the Camino browser for Mac OS X.
FUEL
This is FUEL, a JavaScript library designed to help developers build extensions using terminology and interfaces that are familiar to them.
Rust
The Rust programming language, compiler, standard libraries, tools and documentation.
Servo
The Servo Parallel Browser Project, a protoype web browser engine written in the Rust language.
JavaScript
This is Mozilla JavaScript. It includes SpiderMonkey, Rhino, Epimetheus, as well as debuggers and test suites.
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.
Mozmill Tests
This is repository for the Mozmill tests and API..
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.
amo
This is the addons.mozilla.org website.
Archived Trees
Things below here are present for reference only.
Mozilla Users
This is the home for mozilla.org developers' miscellaneous shared projects.
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)
mobile-browser
This is the Fennec mobile web browser.
Aviary Branch / l10n
This module is Aviary's 1.0.1 Branch, Mozilla toolkit derived projects.
Comm. 2.0
Branch development for Thunderbird, SeaMonkey, and Sunbird development with Gecko 2.0.
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.
Firefox 3
The source specific to the Firefox 3 web browser. Also available is the Firefox 2.0 Branch.
Tamarin
This is Tamarin
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.
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).
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.