Firefox by default doesnt enable some media extensions that are used by html5 video, specifically by youtube.
You can check what is enabled by going here: https://www.youtube.com/html5
Normally some options like mp4 H264 and MSE are disabled, this stops some videos from working, including HD and higher definition videos.
To fix this in Firefox you need to enable a few additional options.
Got to about:config
search for mediasource
In addition to the default settings,
make sure the following are set to true
media.mediasource.mp4.enabled
media.mediasource.enabled
media.mediasource.webm.enabled
Search for mp4
In addition to the default settings,
make sure the following are set to true
media.fragmented-mp4.exposed
media.fragmented-mp4.ffmpeg.enabled
The mp4 settings have changed since I last set them and my browser is slightly different. I didn't need to enable any extra options, but enabled all the mp4 options anyway in addition to the defaults. Mine looks like this
media.fragmented-mp4.enabled;true
media.fragmented-mp4.ffmpeg.enabled;true
media.fragmented-mp4.gmp.enabled;true
media.fragmented-mp4.use-blank-decoder;true
media.mediasource.mp4.enabled;true
@Baz Suggests in another topic to experiment with the media.fragmented-mp4.use-blank-decoder
option as it may or may not cause some issues. Ive had no issues to far with it enabled, but it works with it disabled also.
Check the html5 page again you should have all options enabled: https://www.youtube.com/html5
You should be able to play this video in 4k
note: you may or may not have to install additional codecs depending on your distro.
ffmpeg is a dependency that will likely be required, make sure this is installed. Some distros do not ship with ffmpeg like Fedora, but it is available from rpmfusion in that case.