According to FF3 itself, at least, it's not. Neither is Greasemonkey, Firebug, Platypus, or Rikaichan, all of which I'm waiting to update ASAP as well. But I switched to FF3 RC1 anyway, just because I couldn't stand FF2's ridiculous memory leaks and instability. FF3 RC1 still has some kinks, but overall it looks to be quite an improvement so far.