I assumed that the Cisco H.264 codec is enabled and active after it is downloaded, yet you showed me that there are settings in Firefox that disable it by default ( media.fragmented-mp4.gmp.enabled;false ). So the plug-in gets downloaded automatically and is active but probably never used. Very insightful, thank you.

I did some research on my own in the past and used Vimeo without editing entries in about:config. To make Vimeo work I came to the conclusion that you only need to install gstreamer1.0-libav . As far as I know the Vimeo player doesn't provide technical information about content and by default seems to serve 720p H.264 with the HD option enabled.

Now that I know that I can selectively disable WebM and H.264 in about:config and and verify settings on YouTube's HTML5 page I can do some more tests. I currently only have mobile Internet access on my laptop and I'm only getting 720p as the highest option on YouTube. I will check later when I'm back home (I remember having the 1080p option on my Ubuntu machines without using Flash and without errors).

Okay 1080p playback on YouTube works for me on 14.04 and 15.10. I checked by disabling configuration options for the other codec and checking on the HTML5 player page that they have been disabled. As long as YouTube doesn't stumble over inconsistencies by modifying to much without restarting Firefox everything works fine.

I posted more information in an answer to a very similar question: yt won't work after the ubuntu upgrade