How Firefox Works
The How Firefox Works project is a project for documenting how Firefox does things that may confuse people, such as web developers. One good example would be How Firefox Works/Password Manager#Detection of login boxes, wherein the documentation on the password manager would tell you how it detects what a login box (as this has changed between Firefox 2 and Firefox 3).
This project should alleviate the "read the code" documentation and distribute the documentation workload. Make a request for documentation on a certain element on the talk page.
Contact Tom on IRC if you have any questions.