I also recommend to readers who want to learn key lessons from this massive development project. Again, a good choice.Get the head's up on Mozilla This book couldn't possibly cover the entire Mozilla code, but I think it's excellent at getting you oriented with the structure of the Mozilla code, Mozilla.org and how to use it, basic developer stuff like CVS, and how to build Mozilla on your system, be it Unix, Linux, Windows, or Mac. I rate this one a definite buy.
It's still a good intro if you are planning to dwelve into mozilla's source code, but it's definately lightweight.