Fully Enabling Firefox YouTube Hardware Acceleration

First, check whether your Firefox is even showing acceleration.

about:support

If it says

0/1 Basic

then you basically don’t have it, and you’ll need to do something to turn it on.

Enabling GPU acceleration comes down to two paths, Windows and Linux:

On Windows:

Open about:config

gfx.font_rendering.directwrite.enabled

Set it to True

Also, right-click and create a new integer pref named “mozilla.widget.render-mode” and set it to 6

On Linux:

Open about:config

layers.acceleration.force-enabled
layers.offmainthreadcomposition.enabled

Set both to True

After that you can confirm GPU support is good to go

Next, enable Firefox’s full hardware support for YouTube

Open about:config

media.mediasource.enabled
media.mediasource.webm.enabled
media.mediasource.mp4.enabled
media.fragmented-mp4.exposed
media.fragmented-mp4.ffmpeg.enabled

All True

media.fragmented-mp4.use-blank-decoder

False

Right-click and create a new boolean, media.mediasource.ignore_codecs, set it to True

Restart Firefox, then open https://www.youtube.com/html5 to see whether everything is supported:

Now you can go play a video on YouTube and hardware acceleration will kick in, and some higher resolutions will show up too. Of course, this thing is for watching stuff like Linus Tech blinding you with rich people’s gear; most nukige-type videos are all 720p, sadly…

Addendum:

media.fragmented-mp4.gmp.enable

make absolutely sure this is false