Laz, you're seriously my hero right now.
My computer has been giving me a hard time for quite some time now. Quite a few really annoying things kept happening, the worst of which would be the video card crashing while I was trying to play Browser Based Games (like my beloved Minecraft or BSG) - seeing how I use Firefox, this thread really intrigued me.
The other night I decided to look into this, so I set up my Task Manager to show the memory usage while I was playing around on the computer. Even with nothing happening firefox would continue to soak up more and more memory just like you said.
Normally I leave my computer on 24/7, more often than not Firefox is open with a few tabs. This completely explains why my computer was giving me such a hard time, it was simply running out of resources.
I've since installed this Firemin program, and I've got to say it's working like a charm. I haven't had any issues with crashes since I've started using it.
Thanks for the help, and keep posting these types of threads for us!