I've been using Firefox 3 beta 5 with OS X. I've noticed a few problems -- windows initially appeared almost off screen, one window would abruptly close, but overall it looks like a good incremental improvement. It hasn't pegged my CPU yet, so that's encouraging. (Firefox 2 would do that without any help from Flash.)
Alas, they didn't do anything to tackle the multi-window tab UI problem. I was looking for a view that would render windows and tabs in a browseable outline view with tab names and mouse-over window views, but there's no sign of this.