[padenot]

Would you mind filing a bug at [0] with the content of about:support and maybe a link or two where it doesn't work? Happy to have an initial look into this (I work on the media team at Mozilla).

[0]: https://bugzilla.mozilla.org/enter_bug.cgi?product=Core&comp..., you can log in with a github account or create one

[yoasif_]

I suspect this user is on Linux and doesn't have h.264 codecs installed.

They should be able to do this fairly easily -- two distros here:

https://help.ubuntu.com/community/RestrictedFormats

https://docs.fedoraproject.org/en-US/quick-docs/assembly_ins...