[diggan]

For Firefox:

1. Go to "about:config" in the URL bar and hit Return/Enter

2. In the search box at the top, type in “autoplay”

You'll find two variables to modify. `media.autoplay.enabled` and `media.block-autoplay-until-in-foreground`. They do exactly what they say they do. Turn them off/on as you wish by double-clicking, changing true to false and vice-versa.