Okay, I've experienced this on my laptop (win 8.1) - but didnt know if it was related to a crappy touchpad, or dodgy on-board graphics or FF itself :-)
I think it's okay on my win 7 desktop - but have had lots of troubles with flash/video there (tbh I havent been keeping track, maybe I was having problems with flash/video with win 8 too).
PaleMoon seems to be behaving of late, but it has also broken session manager lately