The guys at the Mozilla Corporation published today a still unfinished guide for all those who wish to develop a extension for Firefox, but don't know where to start. I ignore if a 3rd party did have something similar, but as far as I know, Mozilla just had a rough number of guidelines that seemed not that helpful to a beginner.
This guide changes all of that, and while many things require fixing, it seems quite comprehensive, even including an appendix to help you choose an appropriate license for the resulting extension. Pretty neat.
via Asa Dotzler: Firefox and more