Mozilla Product Manager Alex Polvi has written a brief
overview that explains the current status of the most popular add-ons. Some of my favorites, like Greasemonkey, Firebug, and Stylish are now all compatible with Firefox 3.
Unfortunately, Google Browser Sync is being discontinued and will no longer be supported. Google recommends that users adopt Mozilla Weave or Foxmarks.
Mozilla's add-on team has been hard at work getting a
new version of the add-on web site ready in time for the release. The new site includes a theme browser that displays thumbnails and a new advanced search system, as well as numerous improvements for developers.