Firefox can be a resource hog. On my old computer it would start to kill my system if I left it open for too long, and playing games with it open was a definite no-go. It's not a problem on my new one, but then again, it's a quad-core with 4 gigs of RAM.

It kinda becomes moot if you just close Firefox now and again, though, and make it just re-open your old tabs when you start up.