Stopping HTML5 autoplay in Firefox
Just set media.autoplay.enabled to false (about:config is the page to go). It will stop autoplay in HTML5 videos... which do behave. For those which don't (like YouTube), the next released version of Firefox, ver. 41 set for September 22 release, should solve the problem for good. You actually may get it today as the Firefox 41 Beta (with all the caveats applicable for beta versions).