While Mozilla Firefox has good support for HTML5 standards, you might have noticed that it does not support all features required to play videos on YouTube without having Adobe Flash Player installed. It supports the required HTMLVideoElement, but some videos do not play anyway. Those videos require the Media Source Extensions feature which is disabled by default in Firefox. By default, this option is off in Firefox, so the browser is not able to play some HTML5 video streams. If you prefer HTML5 videos over the Adobe Flash Player, you might want to enable Media Source Extensions to get advanced support of HTML5 videos in Firefox.