firefox quantum - Media opened in a background tab will not play until the tab is selected

This feature was introduced with Firefox 56:

http://mozilla.org/firefox/56.0/releasenotes

However I do not like it. Oftentimes I will middle-click a YouTube song link with the intention that it will open in a background tab and start playing, meanwhile I can continue with whatever I am doing in the current tab.

How can I reverse this new behavior?

Answer

Using the browser console:

Services.prefs.setBoolPref('media.block-autoplay-until-in-foreground', false);

http://wiki.mozilla.org/Firefox/Block_autoplay