A month and a half ago I have tested Firefox 4 to be my primary browser. At the time I tested Firefox 4 beta9pre and beta10pre and it looked good, but there were too many important add-ons missing.
I have tried to repeat the test with Firefox 4 beta13pre today and it looks much better. There are a lot of updated extensions that work with Firefox 4, and there is a specially helpful add-on that enables other, even non compatible add-ons so they can be installed and tested. It’s called Add-on Compatibility Reporter and its current version is 0.8. Besides enabling the add-ons it also takes your feedback whether the add-on works with your version of Firefox or not. For each non-compatible add-on there’s a new ‘Compatibility’ option, where you can select that it works for you or that it’s broken. Very helpful for developers and other users!
I have managed to successfully restore almost all my add-ons so I think I’m very close to changing to FF4 for good!