I just spent 10 minutes trying to install 'cool' extensions to my Firefox browser and not one of the damn things works with the 1.5 RC3 version of the browser!
What's interesting to me is most of them didn't work with the latest version of the released browser, either (1.0.7).
This, I think, is one of the big weakenesses of Firefox. If, whenever a new release comes out, it breaks everyone's extensions (one of the things that allow you to get the same functionality as IE, plus some), well, it's a bitch.
I like componentized software as much as the next guy, more maybe, but the framework's just gotta work better if you want people to switch to Firefox. I'll keep using Firefox (if for nothing else the tabs) but if you break all the things that make it stand out from IE, you give Microsoft the chance to catch up.
Let's hope the Firefox browser folks keep that in mind while developing. In the end, users will use what has the most stability and the most (useful) features.