The decision to include Firefox 3 as standard was a brave but perhaps foolhardy move. A sizeable enough number of extensions are not compatible with FF3 - although I'm pleased that Foxmarks is now available, at least if you sign up to the beta. My usual mouse gestures extension isn't compatible, although I found a replacement. Unfortunately enough, that replacement (FireGestures) was good for 3b4, but not 3b5, which has just been pushed down the pipes, which means that currently I'm having to do without mouse gestures. For me it's no great shakes, as I only have one or two that I use, but if you're a heavy user, it's going to be a problem.
The other issue I've had in Firefox3 is with scaled images. Any images that used height and width tags to scale the image would appear as just a black rectangle - viewing the image full size would work. The issue is described in bug #182038. In summary, it seems there are three possible fixes: xulrunner1.9 has a fix that may or may not work for you; otherwise, try editing your /etc/X11/xorg.conf file and in the "Device" section adding:
Option "AccelMethod" "EXA"
And if that doesn't work (and for me, with SiS onboard graphics, it crashed X), you can try instead adding:
Option "XAANoOffscreenPixmaps" "true"
I've no idea what this does or if it might cause any other problems, but it did the business for me.