How do I set it so that a video keeps playing?

If the video stops as soon as you switch tabs, I’d first check whether it’s the website doing it. Some sites deliberately pause video when the tab loses focus.

If you’re on desktop Firefox, you can also check about:config for media.block-autoplay-until-in-foreground and set it to false.

If it’s something like YouTube and it still pauses, the site may be enforcing its own restriction. Firefox can keep media playing in the background, but the website can decide otherwise.

Thanks Malcolm, but if I do that, won't that mean that every time I open or wake Firefox EVERY video in every tab/window will begin playing? I had that issue before, and fixed that, so maybe this is price I have to pay to avoid shear cocophony...