Now that most of my extensions (with notable exceptions) support FF3 without hax, I'm going to start using it fulltime. My second-favorite* new feature: you can finally drag items in the Bookmarks menu. Yes, Firefox bookmarks have finally (nearly) caught up to IE5 boomarks in usability.
It's the little things that matter.
* My favorite, of course, is the greatly expanded SVG support, so I can continue work on Glass Vellum.