Mozilla Cross Reference
In an open source project, the most important documentation is the code. The mxr is a web application that allows quick searching and browsing Mozilla code.
To the casual user, Bugzilla may look like a drop box for complaints, but in reality, it is the central nervous system of Mozilla development. Nearly all work on Mozilla code starts with a bug report. It doesn't matter if you are fixing an old feature or adding a new one. Without a bug report, your work is invisible to the community.
Mozilla code is constantly changing. Whenever a change is made to the main repository for a Mozilla applications such as Firefox, the code is automatically compiled on all the major operating systems and an error log is generated. Tinderbox is where you can see the results. Here you can find out if your patch has caused any problems. Or you can find the last known revision to build without errors. The main page for trunk builds of Firefox is at http://tinderbox.mozilla.org/Firefox.
Mozilla code is stored in version control. Past revisions of code can be viewed on the web at http://hg.mozilla.org/. Learn how Mozilla uses Mercurial with the Mercurial FAQ at the Mozilla Deverloper Center. Get more in depth instructions by reading The Definitive Guide.