It seems to be originally included in Firefox ver. 49.0.2 (and some lower versions probably): about:config firstname.lastname@example.orgNavigated 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).
Don't bother installing Youtube AutoPlay Stopper no longer works with the new updated Youtube layout for Firefox v18 but I have found another Firefox add-on that does same thing called 'SmartVideo' by Ashish and go to preference then tick the "Save Bandwidth" box and now all the Youtube videos will stop auto playing until you are ready to press play button. Enjoy!
Не работает! Видео останавливает, но звук продолжает играть. Исправьте пожалуйста. Win7, Mozilla Firefox 11.0 ------------------- Does not work! Video stops but the sound continues to play! Fix please. Win7, Mozilla Firefox 11.0