[hackinthebochs]

It simply doesn't work for some non-trivial amount of videos, e.g. some instagram stories or gfycat mp4s. If you load the link directly in the browser it will say the file is corrupt. Loading the same link in chrome plays fine.

[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...