Quite possibly you are asking about YouTube videos here. I can not say I have noticed any easily accessible public explanations of this from Mozilla {Firefox) or Google {YouTube). So if you come across something I would appreciate a link. You tube seems to have switched users to HTML5 instead of FlashPlayer. See * http://www.ghacks.net/2014/07/17/google-enforces-html5-youtube-firefox-33-newer/ * ''Prevent firefox from using HTML5 player in Youtube.'' [/questions/1044212#] [/forums/contributors/711053 .] * Checkout the options and settings at https://www.youtube.com/html5 (Where available this allows switching to FlashPlayer -in some situations) * '''Consider use of the add-on '' YouTube Flash Video Player 35.2 '' <br />https://addons.mozilla.org/en-US/firefox/addon/youtube-flash-video-player/ I would point out that although in some circumstances FlashPlayer will be preferred, it is *Proprietary software, not open source *Often problematic on Firefox ** Tends to be involved in a lot of crashes and hangs ** Is a source of potential security problems. (Has recently had a series of problems and patches) ** Adobe does not support at all, or only partially supports FlashPlayer on some of Firefox's platforms. **May be involved (at least in the past) with issues and conflicts with other plugins. HTML5 is a new web standard. It makes web browsers more powerfull, and is not only involved in playing video * https://en.wikipedia.org/wiki/HTML5