Interesting, same issue here on 7600GT (AGP).
I've started noticing that with 185.xx version and later.
190.18 is quite ok, but still crashed a minute ago - and the interresting thing is that I can recall using FF at that time when crashes occured.
Unfortunately I can't go back to 180.xx because it won't compile on current running kernel.
All the crashes ends up like this:
0: /usr/bin/Xorg(xf86SigHandler+0x79) [0x80cbc79]
2: /usr/lib/xorg/modules//drivers/nvidia_drv.so(_nv001711X+0x2b) [0xb627b31b]
I've attached bug report + old xorg log file.