I have been using Firefox 3 since Beta 4 and I love it so far. The better memory management made it very much worth the few crashes that happened due to the Beta status. If you have been thinking of upgrading to Firefox 3 then now is probably a good time because Mozilla released the Release Candidate 1 over the weekend (Saturday, May 17). Obviously I upgraded right away, and so far have found it to be very stable.
Most of the extensions aren’t officially supported, but I have found that many work without any changes. You can easily get around the version check by setting extensions.checkCompatibility to false.