Windows 7 x64, Dell Optiplex 990, 3 Dell monitors, everything else stock.
Two monitors are running off of the built in VGA and Display Port. I originally installed a Geforce 6200 to get the third monitor up. Works fine in Chrome and nothing else is slow. (office pc, so no gaming tests I suppose)
Firefox was REALLY slow and had blacked out parts unless you hovered over them. So I got rid of the card completely and installed/plugged in an EVGA UV+ 19. Now Firefox is great on the monitors that are NOT on the UV+, but slow and laggy on the one that is.
Drivers and Windows updates are current. Firefox is the latest, version 12.0, no extensions, just a brand new install of it. Safe mode does not help.