I had this problem, I figured out a fix, but I can't find a better place to say "Hey, if you have this problem, do this" Fairly recently, I had a massive computer update- I hadn't updated my computer in a long time, and somehow the "Auto download and install setting" got turned back on (it's off because I want to know what's going on with my computer, so I'm probably glaring at some tech support people) and it forced it through. For some reason, it almost completely broke FireFox. At first, I thought FF had updated on it's own, as most of my add-ons were turned off, and would not turn back on. I had to switch to the experimental version of Brief to get it to work. Then the prompt to update FireFox popped up. So I eventually restored my computer back to a day or so before the update, ran my computer like that for a few days, and then restored back to the day I first ran the system restore trying to fix the problem. I will be the first to say I don't know why it worked, but it did. Hopefully this will help other people.