When installing firefox from the official Arch repos ffmpeg4.4 is the actual dependency needed by firefox in order to be able to play H264/HEVC/AAC encoded videos, installing the latest version of ffmpeg instead will just break video playback leading to MIME format errors.
Resources:
https://archlinux.org/packages/extra/x86_64/firefox/
When installing firefox from the official Arch repos ffmpeg4.4 is the actual dependency needed by firefox in order to be able to play H264/HEVC/AAC encoded videos, installing the latest version of ffmpeg instead will just break video playback leading to MIME format errors.
Resources:
https://archlinux.org/packages/extra/x86_64/firefox/