Yeah, the compatibility changes between 3.6 and 4 really bit some add-on authors. Extensions now need a manifest (which does reduce startup time), and there are some significant internal changes. I know my own extensions like OverbiteFF needed to be overhauled somewhat, and when Electrolysis comes, there will be another cathartic API gap. I'm hoping Electrolysis comes later than sooner, because I'm concerned it may make Mozilla no longer buildable on Tiger (among other things, it depends on Chromium IPC and IPC currently requires linking everything into a big fat libxul which makes the Tiger linker barf).