Preloaded firefox bundle missing codecs

you can install a browser from flathub with flatpak. you can check a browsers codec support at https://html5test.com/
you can install ffmpeg with bash script from here. you can also try to install ffmpeg from source, see here
the vlc bundle is also missing codecs, and you can also install that with flatpak

This is expected and documented. See: FAQ — Documentation for Clear Linux* project

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