Mozilla

Firefox, XUL, XBL, XPCOM, Javascript, CSS, ...

Platform Development Cheat Sheet: Common commands and resources used for developing Mozilla related technologies.
New to Mozilla: Quick tips to get you on your way to becoming a mozillian.
XPCOM Tutorial: XPCOM tutorial based on Mozilla 2.0 (and 5.0, 6.0, 7.0, 8.0, 9.0, 10.0) (FF4+).
XUL Runner App Tutorial: XUL Runner tutorial based on Mozilla 2.0, (and 5.0, 6.0, 7.0, 8.0, 9.0, 10.0) (FF4+).
Extension Tutorial: A tutorial on how extensions work specifically with Mozilla 2.0, (and 5.0, 6.0, 7.0, 8.0, 9.0, 10.0) (FF4+).