Personally, I have 5-6 'must-have addons' that keep me on Firefox, but they've all now been updated for the new API. The only perhaps outstanding breakage is add-ons that implement alternative tab bars (I use a couple to put the tab bar on the side) -- they still are somewhat limited in how they can customize the sidebar, and the add-on now has no interface to disable the default tab bar, so you need to do that yourself with userChrome.css. As I understand, add-on devs are working with FF to address this soon.