Why does firefox go fullscreen randomly on its own?

For at least a few months now, my browser will suddenly and randomly go fullscreen all by itself. I can't exit it, and then it goes back to normal after at most a minute. It hasn't happened in at least a week. But today, it froze like that, not responding. I quit firefox, waited a bit, then restarted it. It went fullscreen and froze again. I waited at least 10min and it then went back to normal and responded again. It only happens when using firefox. I've done virus scans, etc, and it still keeps happening. Please help. Thanks.