I was really happy when I discovered I wasn't the only person with this problem. It has been hitting me periodically. Now I know why. Hard drive space or rather the lack thereof. With 8GB of space left on my hard drive, Safari ran find but Firefox did not. I did a quick search of files on my drive over 50MB and found tons of junk I could remove which included cache files of various sorts, but was mostly standard files I just didn't need anymore. In the end, I ended up with 25GB of free space on the drive and all my Firefox problems vanished. I didn't even have to reboot. I'm a power user when it comes to Firefox. I think it takes a lot of memory. When it runs out of space to operate in, it writes to the hard drive continuously for every action performed. Thus the nightmarish pinwheel of insanity ensues. So make lots of room on your hard drive. I'd recommend at least 20GB of free space at least. Cheers!