This is a change on the mobile YouTube site. Browsers have a feature that sites can use to know when the page has lost focus or the screen is turned off: https://developer.mozilla.org/docs/Web/API/Page_Visibility_API Someone recently posted an add-on that apparently prevents websites from detecting that event. It's called: [https://addons.mozilla.org/firefox/addon/video-background-play-fix/ Video Background Play Fix] and has this description: <blockquote>Some sites may not work with Firefox video background play feature. This add-on provides a quick fix by blocking the Page Visibility API and the Fullscreen API.</blockquote> I haven't tried it myself, but the poster in this thread said it solves this issue: [https://support.mozilla.org/questions/1159048 Please let keep firefox playing on when the main screen is off]. Caution: Pages may use more data and battery than normal if they can't pause when you switch tabs or turn off the screen.