Last update, just want to get all the facts out, its pretty damn serious, was jus reading the blog of the guy that exposed it, hes got a few expamples up, that aren't harmful but jus to show it works, they look like .mp3 files to me, even says .mp3 in the status bar when moving mouse over, can't tell at all it is actually a .qtl file in disguise, this is veryy easily exploitable if people start using it for bad which of course people will, so thats it no quicktime for me til they fix this.
Oh, AND it affects IE too, tho not as critically, so its not a Firefox bug but almost entirely a Quicktime one.
Quicktime the ONLY Apple product i can even stand, lol, cause I like watching movie trailers in awesome HD quality, but, flash video fine for me for a while.