I’ve migrated several of my machines over to Firefox 3 RC3 and I like it.  Of course, this means that some of the old Add-ons need replacing/upgrading.  My recommended Add-ons:

• Delicious Bookmarks:  Essential for a del.icio.us linkblogger like me.  The 2.x version adds a few active icons to the status bar.
• Gmail Notifier:  Needs no introduction.  If you use Gmail, you need this.
• OPML Support:  When I want to port my bookmarks between FF instances, this is clean.  I rarely use it.
• QuickProxy:  Essential on my work laptop.  I used to use SwitchProxy but found this one when upgrading to FF3.  It doesn’t allow multiple proxy configurations, but I don’t need more and its one-click on/off operation makes up for it.  SwitchProxy hasn’t been upgraded to a FF3-compatible version yet.