Firefox 3 has reached release candidate status, which means that it shouldn't be too long before general release.
There are a few add-ons that are not quite ready for Firefox 3 at the moment, including FireBug, an excellent web development tool set. There is a beta version of FireBug that does work with Firefox 3, hopefully the final version will be available soon.
Firefox 3 Release Candidate 1 (recommended for testing purposes only).