I have spent the last few days hacking a Firefox extension. While the platform is pretty cool and extensible the documentation is sparse to say the least. Even searching the web the amount of information is very sparse. This is proably an indication of the platform maturity.
However the number of extensions is however growing (a very good sign), so learning by readiing code demonstrates a key to the success of open source - building on the shoulders of giants.
With a little effort I was able to hack the webdeveloper extension and add a few new features that some of our design team needed. Many thanks to Chris Pederick for all his hard work and making his tools available.
It is really nice to be able to build on a full HTML/CSS rendering engine. I look forward to seeing a lot of great applications come from the platform. Once full EX4 support arrives with 1.1 it should be even better.