Well, I'm somewhat gobsmacked and amazed to find that I'm the first person posting about this, but Firefox 4 is now final and available for download. Distros like openSUSE11.4 should provide updates from the beta (haven't looked yet), and I'm sure there will be PPAs for *Ubuntus.
There's a tour of the new browser's features here.
Of course, Firefox is not just good for Linux, it's good for other OSes too, and I'll be updating my Mac very shortly.