Open about:config
Search for media*play
Set media.block-autoplay-until-in-foreground;false
Set media.autoplay.enabled;false
Disable Autoplay in Firefox
Type about:config into the URL bar, and confirm you'll be careful by clicking the I'll be careful, I promise button. 2. Search for the string labeled media.autoplay and double-click it to flip its status to off. Opera and Chrome have an extension that prevents videos from playing automatically.