I did some testing with tc-15.x x86 and firefox-111.01
Using Xorg-7.7 flwm aterm wbar: firefox
[GFX1-]: glxtest: libpci missing
[GFX1-]: glxtest: libEGL initialize failed
[GFX1-]: glxtest: GLX extension missing
[GFX1-]: No GPUs detected via PCI
...
play random youtube video
...
[Child 5557, MediaDecoderStateMachine #1] WARNING: Decoder=a4955da0 Decode error: NS_ERROR_DOM_MEDIA_FATAL_ERR (0x806e0005) - Error no decoder found for audio/mp4a-latm: file /builds/worker/checkouts/gecko/dom/media/MediaDecoderStateMachineBase.cpp:164
[Child 5557, MediaDecoderStateMachine #1] WARNING: Decoder=a4955a60 Decode error: NS_ERROR_DOM_MEDIA_FATAL_ERR (0x806e0005) - Error no decoder found for audio/mp4a-latm: file /builds/worker/checkouts/gecko/dom/media/MediaDecoderStateMachineBase.cpp:164
[Child 5557, MediaDecoderStateMachine #1] WARNING: Decoder=a4955f40 Decode error: NS_ERROR_DOM_MEDIA_FATAL_ERR (0x806e0005) - Error no decoder found for audio/mp4a-latm: file /builds/worker/checkouts/gecko/dom/media/MediaDecoderStateMachineBase.cpp:164
[Child 5557, MediaDecoderStateMachine #1] WARNING: Decoder=a4956280 Decode error: NS_ERROR_DOM_MEDIA_FATAL_ERR (0x806e0005) - Error no decoder found for audio/mp4a-latm: file /builds/worker/checkouts/gecko/dom/media/MediaDecoderStateMachineBase.cpp:164
[Child 5557, MediaDecoderStateMachine #1] WARNING: a31cf040 OpenCubeb() failed to init cubeb: file /builds/worker/checkouts/gecko/dom/media/AudioStream.cpp:281
[Child 5557, MediaDecoderStateMachine #1] WARNING: Decoder=9f416b00 [OnMediaSinkAudioError]: file /builds/worker/checkouts/gecko/dom/media/MediaDecoderStateMachine.cpp:4550
[video plays without sound]
Loading above plus pci-utils xf86-video-intel Xorg-7.7-3d alsa-config alsa pavucontrol and using pavucontrol to unmuute sound: firefox
[GFX1-]: glxtest: VA-API test failed: missing or old libva-drm library.
ATTENTION: default value of option mesa_glthread overridden by environment
...
[play random youtube video]
...
[Child 9552, MediaDecoderStateMachine #1] WARNING: Decoder=a44548c0 Decode error: NS_ERROR_DOM_MEDIA_FATAL_ERR (0x806e0005) - Error no decoder found for audio/mp4a-latm: file /builds/worker/checkouts/gecko/dom/media/MediaDecoderStateMachineBase.cpp:164
[Child 9552, MediaDecoderStateMachine #1] WARNING: Decoder=a4454720 Decode error: NS_ERROR_DOM_MEDIA_FATAL_ERR (0x806e0005) - Error no decoder found for audio/mp4a-latm: file /builds/worker/checkouts/gecko/dom/media/MediaDecoderStateMachineBase.cpp:164
[Child 9552, MediaDecoderStateMachine #1] WARNING: Decoder=a4454a60 Decode error: NS_ERROR_DOM_MEDIA_FATAL_ERR (0x806e0005) - Error no decoder found for audio/mp4a-latm: file /builds/worker/checkouts/gecko/dom/media/MediaDecoderStateMachineBase.cpp:164
[Child 9552, MediaDecoderStateMachine #1] WARNING: Decoder=a4454c00 Decode error: NS_ERROR_DOM_MEDIA_FATAL_ERR (0x806e0005) - Error no decoder found for audio/mp4a-latm: file /builds/worker/checkouts/gecko/dom/media/MediaDecoderStateMachineBase.cpp:164
ATTENTION: default value of option mesa_glthread overridden by environment.
...
[video plays with sound]
loading intel-vaapi-driver removes the VA-API warning.