Mercurial (also known as "hg"), is the distributed version control software used for the development of Firefox, Thunderbird, and the shared Gecko core. It replaced CVS after Mozilla 1.9 was branched.
hg is the Mercurial command-line tool, Hg being the chemical symbol for the element mercury.
First, read every word of Mercurial basics, before you do anything else.
Then, the Mercurial FAQ is a decent place to start.
See Getting Mozilla Source Code Using Mercurial for getting a tree to build.
See Mercurial Queues for managing queues of patches, and how to integrate Mercurial with Bugzilla.
The hg book is the definitive Mercurial user guide.
The Mercurial tag lists the Mercurial-related articles on MDC.
And on wiki.mozilla.org, these helpful pages:
- Using Mercurial locally with CVS, a how-to. (Note that this is only useful for code that's still exclusively in CVS.)
- Mercurial on Windows
- Publishing Mercurial Clones - If you have a LDAP account that allows you to push to hg.mozilla.org you can also create your own user repositories on the server to share work.