either the fedora, or the flathub build of firefox didnt come with some video codec, OpenH264 i think. switching to the other build fixed it (imo more a licensing issue with the codec than a flatpak problem)
Just in case anyone in this thread also has problems with video playback on flathub Firefox, I just solved that by installing the ffmpeg-full flatpak.
No idea why a dependency that is needed to play video without jitter isn’t installed automatically.