I've been having a strange issue happen me for a while and I'm unsure why it happens.
In short, when I move my cursor off of a YouTube video the video freezes; however, the audio keeps playing perfectly fine. (Basically 0fps video)
When I bring my cursor back on top of the video, it goes to what should be the current frame and resumes its normal fps.
This does not happen with every video all the time, it seems somewhat random when this occurs.
I noticed it's more prone to happen if I move my cursor from the screen the video is playing on onto another monitor and/or when I drag the video tab out of the active FireFox window (making the video its own window) and place it on another monitor.
Bringing my cursor back onto the screen the video is playing on does not fix the issue, I have to move the cursor directly on top of the embedded video player for it to resume as normal.