The Firefox 3.5 has now been out for a week, with some good reports. According to all the countless reviews there are many improvements. I think my favourite will be to restore browser windows I accidently closed! Or restore when the brower managed to crash. (You can restore without incuding what you think caused the crash if necessary.) Also "word prediction" and Geo location (coming up with suggestions based on where I am) may be useful.
Lifehack list of new features