My least-favorite bug still isn't fixed. Open up several dozen tabs full of image files and the display goes all wonky. I think Moz is using video memory for all of it. At least, I get the problem a lot sooner on PCs with less-capable video cards.
Of course, no other browser I'm aware of can handle having 80 or so open windows each containing ~ 10 600x400 JPGs, so maybe I shouldn't complain too loudly.
Also, Moz v 1.4 is the same thing as RC3, so if you're using that, don't bother downloading.