User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:42.0) Gecko/20100101 Firefox/42.0 Build ID: 20151030084315 Steps to reproduce: Although Version 42 notes said both should be enabled, they aren't in two computers with Ubuntu I have tested. Actual results: Youtube videos play in Flash, and if you enable HTML5 its poor and only in 360p and 720p . Expected results: They should be enabled, in About:Config if you enable both of them they works excellent.
I also see media.mediasource.enabled -> false in Firefox 42, using the firefox package in Ubuntu 15.10 64-bit. Videos were playing in 360p instead of 4K.
It's enabled in 43. If you still only get 360p, blame YouTube.
Note that if you change the user agent to report version 44 everything will work as you expect.