I’ve been running it for all of 15 minutes or so. I’ve observed no radical changes yet, which is fine, since in my view there wasn’t anything egregiously wrong with Firefox to begin with. Overall it looks like they’ve integrated some of the best ideas arising from the extensions developed for 1.x: finer-grained tab control and session persistence, to name but two.
My favorite new feature so far is integration with third-party RSS readers: it’s now possible for me to visit a site with Firefox, and then add its feed to my NetNewsWire subscriptions with two clicks. Very nice indeed.