As of a few Firefox updates ago, and/or my update to Fedora Core 6, I've had all sorts of stability problems with Firefox on my Linux system. I'm fairly certain it's related to Flash Player (v9). I've tried reinstalling everything (including Firefox and Flash), as well as completely nuking my FF profiles. Neither has helped.... I am using the proprietary nVidia drivers for my X11 server.

Looking around for something different, I ran into something that sounded similar to what my problem is. I honestly don't know what it does, but adding the following to my Firefox startup script seems to help:

export XLIB_SKIP_ARGB_VISUALS=1

Another option is to disable the "Composite" feature in your X11 (Xorg) configuration.