New install of Ubuntu 15.04 (on an otherwise fully functioning dell XPS 13 9343). I've been having difficulty getting HTML5 video to play on Firefox.

My test case is Plex and Youtube, both of which seem to default to using flash player (urgh, I would very much like to banish it!).

If I use Chrome (not so great as I have a high DPI screen) on Ubuntu the html5 player works fine for youtube and plex. Similarly if I boot into windows plex and youtube use the html5 player on both chrome and Firefox.

I therefore assume I’m missing a video codec or Firefox setting. Ubuntu restricted extras are installed.

Can anyone recommend a place to start looking

  • you have to disable the extension in your firefox browser for html5 to be the default.
    – Virusboy
    May 15, 2015 at 17:54
  • which extension would that be? Could you go into further detail?
    – Huw
    May 15, 2015 at 18:08
  • I believe I have found a lead relating to media.fragmented-mp4.* configuration settings but this is causing plex to hang. Will post an answer if I crack it, but i'd appreciate any specific guidance
    – Huw
    May 15, 2015 at 18:52
  • This post on Phronix seems a good start. It gets me to a full roster of tick boxes on the youtube html 5 check page but still no joy on plex. Noting this paragraph: Currently MSE-enabled versions of Firefox default to using the lowest quality available, though it can be forced to use a higher-quality by specifying one using the gear settings for the video. This issue is due to this [bug](https://bugzilla.mozilla.org/show_bug.cgi?id=238041).
    – Huw
    May 18, 2015 at 16:40

2 Answers 2


you could try this which worked for me:

in firefox enter the about:config page.

find the entries specified below and set them as I've set them below whichs did the trick for me:

media.mediasource.whitelist = false and media.mediasource.webm.enabled = true

Regards, Troels


Try using this addon for firefox:


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