I'm experiencing this same problem, in both Firefox 3 and Firefox 2. Any time I'm scrolling, the screen flashes black momentarily, and will occasionally completely lock X other than the mouse, forcing a reboot of X if Ctrl-Alt-Backspace works, and a hard reboot otherwise (it's random).
In addition, in 3D games, it seems as if there is "jitter", by which I mean that it seems to render for a few frames, then show a frame from a second ago, then continue rendering. This loops.
I get the same slow terminal resize performance.
GeForce 7700, Gentoo Linux (Portage drivers), Core 2 Duo T7200, 2 GB RAM. I'm using Compiz-Fusion but I believe this persists even in Metacity.
Here's my bug report log.