All browsers save info, whether to user files, a centralized "registry", or to .plists. Some even save the stuff to evilbad hidden death files that kill you when you think about them.
I don't think Firefox falls into the category of browser that uses the hidden death files. You're safe to think about these files regarding Firefox, I doubt that the Hidden File Ninjas will come out from behind your can of Diet Dr. Pepper and cut your throat with discarded CDRs.
Besides, who cares? The hidden file thing is so 1998. Seriously... THERE'S THINGS OTHER THAN THE HIDDEN FILES!