I recently downloaded the latest security update for my Macbook Air running Sierra, and everything seemed fine. Then while watching a 2 hour video in fullscreen mode at 1080p60FPS, the video suddenly froze. The audio continued, so I tried exiting out of full screen, but my cursor wouldn't move. I couldn't press escape, or pause the video, or force quit Firefox, but the video was still playing fine. I hard reset my computer and the video resumed and everything was normal again. The problem went away but just a few minutes ago while watching another video at 720p60FPS in fullscreen the video began to lag. I managed to move my cursor, which was very laggy, and exit out of full screen. After a few seconds of laggy cursor, I clicked out of firefox, and my computer began running normally again, the video was fixed and everything was smooth. I checked activity monitor and it told me everything is fine. Is this a problem with my Mac, or a problem with Firefox and Youtube? As I said I've never had this issue on this computer and I've had it more than a year, and I've used Firefox the whole time. I only run into this issue at night after using my computer all day, and my computer has felt slightly warmer than usual lately, but nothing to the point of needing to worry. I don't have Time Machine set up because I've never had the need to use it.