When the screen rotates, many videos freeze and thereafter all videos become unresponsive (and even some elements of the Firefox UI stop responding to taps) until the application is closed and restarted. I have experienced this problem for at least a year now. Reddit is a repeatable example: going to any feed (such as r/popular with videos), playing one, and rotating the screen.

I turned off all battery and CPU optimizations for Firefox, which improves performance on videos somewhat, but does not resolve this issue.