Are you sure you are talking about Linux? Those pages mention mostly Windows and say nothing about video at all.

When I go to about:support I see "AzureCanvasAccelerated: 0" and "HW_COMPOSITING: Acceleration blocked by platform".

Setting layers.acceleration.force-enabled to true and restarting FF does not change that.

I tried it on 3 different machines with different graphic cards and different drivers and see the same behaviour on all 3.

Yes I am sure,

https://askubuntu.com/questions/491750/force-enable-hardware...

layers.acceleration.force-enabled overrides "Acceleration blocked by platform" (you should see "Force-enabled by pref" right under it).

But this indeed has nothing to do with video! This is layer compositing.

Sure it does, because it is part of the GPU hardware acceleration stack.

https://askubuntu.com/questions/491750/force-enable-hardware...

On my case for the EEPC 1215B Brazos APU it worked, like everything in Linux there isn't a solution that makes all desktop users happy.

GPU hardware acceleration has nothing to do with video decode. (Okay, the hardware is often on the same physical silicon die, but they're separate things in software.)

There is literally no support yet for any unix video decode APIs in Firefox. No VA-API, no VDPAU, no V4L2, no nothing.

Here's the bug for VAAPI: https://bugzilla.mozilla.org/show_bug.cgi?id=1210727

There's a bounty for it: https://www.bountysource.com/issues/55506502-add-va-api-hard...

Understood, then why did it help reducing the CPU load on rendering?

Because if you're compositing on the GPU, you're not compositing on the CPU :) It shouldn't help that much with video players, but it does always help a little