Only had two problems with addons - Deepest Sender and SubmitToTab are not compatible with 3.6 yet. IETab has been replaced by Coral IE Tab which includes Adblock support, which is nice. Oh, and I got rid of HideMenuBar, as that functionality is now built in (right click on the menu bar and untick it - it comes back when you press alt).
I can live without Deepest Sender (but would appreciate knowing if there's another FF addon that does lj updates) - but I really miss being able to submit forms to a new tab - so if anyone knows of an alternate addon then let me know!