carl,
if it happens again, just keep pressing the close on all those error messages. they will eventually stop. i think i got 44 or 54 of them once. i forget, but it was a lot.
then, if FF still seems to be running...somewhere... , hit 'control-alt-delete' to call up the task manager, find FF, highlight it if it's not already, and hit 'end process' (or end task). that shld clear it.
then, it's recommended that you reboot your machine. your resources will be low and a reboot shld restore everything.
you're still going to lose that one filter as being corrupted. so, like steve said, when you see those early warning signs, save and kill FF and start again.
also, i have noticed since upgrading my 'critical updates' for windows, including service pack 2, i.e. 7, windows media upgrades and some other things, that this hasnt happened AT ALL on my machine, where i was getting it regularly before. so, this all points to a very large memory leak within windows xp itself, and not filter forge. filter forge is just the victim here and not the problem itself.
If wishes were horses... there'd be a whole lot of horse crap to clean up!
Craig