Videos pause when Firefox isn't the focus, most often I'll have it playing on my second monitor when doing something else. I find this most often happens with streams but not exclusively. I also find that it's more likely to happen when I have a game in the focus on my main monitor. If I have the game open but not in focus I find that this issue happens significantly less (even if that doesn't significantly change the impact the game has on my computer). This is a new issue that appeared sometime within the last month or so and wasn't there for years prior.