Are You In The Right Place?
Want to hack on Gecko? The Developer Guide is probably what you are looking for.
Feature Planning / Specs
Most feature planning should happen in Bugzilla, but for some things it's easier to map out a spec on the wiki first.
Contributing to the Mozilla codebase If you're just beginning.
Frame inheritance hierarchy (dated May 18, 2008)
Mozilla's Layout Engine (Presentation by L. David Baron) (dated Thursday, December 14, 2006, 13:30 PST
Gecko:Miscellaneous Notes Random documentation we don't yet know how to categorize.
Gecko:Task List If you're familiar with Gecko code and want to contribute.
Gecko:Testing Tasks if you're interested in getting started by helping with testing Gecko
Gecko:Overview work-in-progress documentation giving an overview of how Gecko works
Gecko:Frequently Asked Questions Developers' FAQ.
Gecko:Wish List Architecture and implementation ideas.
XUL development happens at XUL:Home Page.