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.