Build in Firefox Rated 3 out of 5 stars

It seems to be originally included in Firefox ver. 49.0.2 (and some lower versions probably):
about:config
extensions.youtubeautoplaystopper@grizzlyape.comNavigated
But setting true/false seems to do nothing...

Another command built-in Firefox:
media.autoplay.enabled
false - always stop autoplay
true - video opened in new tab is buffered only in background and start play automatically only after you view the tab. Opened in the same tab start play automatically.

Last way is use addon YouTube ALL HTML5 which have option to always stop autoplay or stop only in background (work like media.autoplay.enabled but not play automatically, play button must be pressed).