Playing a specific video on Youtube now results in "An error occurred..." message. I was able to play the video 2 weeks ago. It is only doing this at this video and not of any other video of "The script". I'm using Firefox 40.0.3 for Ubuntu Linux with Flash disabled and use the HTML5 player. Playback only results in a error message with this particularly video: https://www.youtube.com/watch?v=ho9xM9n2USA This is what I did so far: - Enable Flash to see if it can play in Flash: No does not work - Restart with deactivated : does not work - Switch Firefox profile (firefox -p). This would rule out all cookie and plugin issues: no does not work Meanwhile I can pinpoint this to Firefox for Linux: - Ubuntu standard Internet browser DOES play the video - The Google Chromium browser in Linux also plays the video - I can play the video using Firefox for Windows. So, it must be in this version 40.0.3 for Linux. I use www.youtube.com/html5 to check firefox on how it can playback HTML5 content. It CAN play back: - HTMLVideoElement - H.264 - WebM VP8 It CANNOT play: - Media Source Extensions - MSE & H.264 - MSE & WebM VP9 So if I do now the same test on Firefox on Windows where the video plays, 2 of them are positive. "Media Source Extensions" and "MSE & H.264". So one of both must be the cause that it is not working on Firefox on Linux. Using this information I switched Firefox parameters: http://www.ghacks.net/2014/07/25/enable-mse-h2-64-support-youtube-firefox-right-now/ enter: about:config media.mediasource.enabled > TRUE Using this I could enable "Media Source Extensions". After switching this I got my video WORKING !!!! media.mediasource.mp4.enabled (already was true) media.fragmented-mp4.enabled (already was true) media.fragmented-mp4.exposed > TRUE media.fragmented-mp4.ffmpeg.enabled > TRUE media.fragmented-mp4.gmp.enabled > TRUE media.fragmented-mp4.use-blank-decoder (already was false and must be false) This is a nice solution for me but when is this solved in the standard version? Nobody is working on this bug: Bug 778617 - (MSE) Tracking bug for Media Source Extensions API implementation. New versions of Firefox could flip back the parameters. For now it works, but for how long?