HTML5 does not work in YouTube
I use Linux and cannot get Adobe Flash to work anymore so we want to use HTML5. As recommended in Firefox I added that and no youtube videos work even tho it says HTML5 is used when it can be.
Wšě wotmołwy (1)
If the problem is limited to YouTube, perhaps one or more of your extensions override YouTube's default? If you right-click the player and check the context menu, the last line usually indicates Flash or HTML5.
In order to decode MP4 video, Firefox on Linux uses GStreamer. You could use the following page to test whether that is working in general: