https://developer.mozilla.org/en-US/docs/Mozilla/Developer_g...

Code Firefox, a series of videos demonstrating how to download the code and build it (which, btw, takes less than ten minutes for a full clean build on my three-year-old MacBook Pro :)

http://codefirefox.org/

Bugs Ahoy!, a Bugzilla search engine for "good first bugs" with a mentor's contact information, filtered by programming language or feature area matching your interests:

https://wiki.mozilla.org/BugsAhoy

There are also friendly Mozilla developers on the #introduction IRC channel on Mozilla's IRC server ready to answer new contributors' questions.