There's a hidden setting for this: (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button promising to be careful. (2) In the search box above the list, type or paste '''autop''' and pause while the list is filtered (3) Double-click the '''media.block-autoplay-until-in-foreground''' preference to switch the value from false to true That probably should be the default...

thanks