Although we’ve actually been working on Gecko 1.9.3 documentation for a while now, we finally now know that these changes will be in Firefox 4. As such, I’ve now set up the Firefox 4 for developers page on the Mozilla Developer Center site; this page will provide links to all Firefox 4 developer documentation.
A few articles have already been written. There’s lots left to do, of course, but we’re making progress. If you see something you’d like to write about, feel free to contribute!
As always, the content of that page is in flux while the feature set of Firefox 4 continues to be adjusted. In addition, it’s obviously too early to be sure exactly what articles will be produced for all the topics at hand, so there are a number of placeholders.
If you’ve contributed code to Gecko or Firefox recently, you may be hearing from me or one of the documentation contributors soon, to gather information about your area of expertise as documentation comes together.