If I go to a YouTube video and open it to full screen, it will play for a couple of seconds before leaving full screen while still playing. It will only do this once, if I again go to full screen it will stay full screen. This happens with almost every time I play a YouTube video (~95% of the time). This happens if the video is already playing, or if it's still loading. I have refreshed Firefox and reinstalled. I have updated Windows. I have no add-ons, extensions or themes installed. Disabling hardware acceleration does not change anything. If I run in safe mode the problem goes away, though it runs slow and has other problems.