Well, this was driving me nuts. After hours of trying to get a dual monitor setup with my new board and Geforce card, now Flash kept fucking up. The answer I found seemed simple:
1. Set bpp in /etc/X11/xorg.conf to 24.
That's because Flash needs 24bpp? Weird. But won't work for me because I need my Voodoo4 to be at 16bpp.
2. Option "Composite" "0" in xorg.conf
Ok. Sure. I don't know exactly what it does anyway. But restarting X makes me nervous. (All those hours wasted....) Glad some guy left an easier answer somewhere:
3. export XLIB_SKIP_ARGB_VISUALS=1 before running firefox
I found this last advice here. Worked for me.