It seems like hardware accelaration decoding is currently disabled
about:support
It seems like this error comes from these few lines:
https://searchfox.org/mozilla-release/source/widget/gtk/GfxInfo.cpp
// Disable on Release/late Beta
#if !defined(EARLY_BETA_OR_EARLIER)
APPEND_TO_DRIVER_BLOCKLIST(OperatingSystem::Linux, DeviceFamily::All,
nsIGfxInfo::FEATURE_HARDWARE_VIDEO_DECODING,
nsIGfxInfo::FEATURE_BLOCKED_DEVICE,
DRIVER_COMPARISON_IGNORED, V(0, 0, 0, 0),
"FEATURE_HARDWARE_VIDEO_DECODING_DISABLE", "");
Fedora patches this: Tree - rpms/firefox - src.fedoraproject.org
And some other patches aswell: Tree - rpms/firefox - src.fedoraproject.org
Could this be also be patched for the firefox package?
There is a thread in the arch forums:
https://bbs.archlinux.org/viewtopic.php?id=281398
They claim, you can add this entry:
media.hardware-video-decoding.force-enabled=true
to about:config and it should reenable vaapi.
@mithrial
Seems the OP already used that or other pref if you look at the image posted, the second line in middle… 
I can’t see or read images.
Does Manjaro have their own Firefox packages or is it inherited from arch? With the first one, it would be easy to patch.
Otherwise, the target for creating the issue is arch’s mailing list.
Edit: I do have the same output in about:support but when using intel_gpu_top and playing a video, I can see that the GPU is in fact used.
That entry is already there, just marked as false. But media.hardware-video-decoding-enabled is marked as True
Oh I made a stupid mistake:
But about:support still shows:
I guess it is working now: I have log entries containing " VA-API FFmpeg init successful"
Thats new
For nvidia + libva-nvidia-driver you must set
widget.dmabuf.force-enabled=true in about:config
with or without Wayland
