I have seen the previous post on the ffmpeg issue (https://forum.artixlinux.org/index.php/topic,5191.0.html) and it is marked as resolved but it does not look like that to me. I have been quite busy this weekend so I had not have time to debug it but apparently people have rolled patches for it (https://gitlab.com/librewolf-community/browser/arch/-/issues/102#note_1310308421) that should just work.
I noticed it when mp4 videos and gifs won't load and error out with message
[Child 30350, MediaDecoderStateMachine #1] WARNING: Decoder=7fd0b0da4100 Decode error: NS_ERROR_DOM_MEDIA_FATAL_ERR (0x806e0005) - Error no decoder found for audio/mp4a-latm: file /build/librewolf/src/firefox-110.0/dom/media/MediaDecoderStateMachineBase.cpp:164
[Child 30350, MediaDecoderStateMachine #1] WARNING: Decoder=7fd0b0da4100 Decode error: NS_ERROR_DOM_MEDIA_FATAL_ERR (0x806e0005) - Error no decoder found for video/avc: file /build/librewolf/src/firefox-110.0/dom/media/MediaDecoderStateMachineBase.cpp:164
It is fixed when I installed the new librewolf-bin from the AUR which just had these patches applied (https://aur.archlinux.org/cgit/aur.git/commit/?h=librewolf&id=af9495e31b180a3ea223a789ba93f387b0aeaaaa)
The errors are from 110.0-3, I noticed the upgrade to that yesterday and saw that post. It seems to be fixed in 110.0.1-1 though.