You need the ffmpeg-full platform extension. flatpak install org.freedesktop.Platform.ffmpeg-full/x86_64. Note that if there are multiple ffmpeg-full extensions available, you must choose the same version that firefox uses for its runtime. As of november 2021 that's the 21.08 runtime, so the command would be flatpak install org.freedesktop.Platform.ffmpeg-full/x86_64/21.08. You may also want to pin it (to avoid it getting removed) with flatpak pin runtime/org.freedesktop.Platform.ffmpeg-full/x86_64/21.08.