MP4 is just a container, the specific audio/video streams can be one of several different codecs, and if you don't have the codec used it won't work. If you can identify the encoding you could probably just download a codec and be good to go.

Edit: for this video the video codec is

Codec: MPEG-H Part2/HEVC (H.265) (hvc1)

and audio codec is

Codec: MPEG AAC Audio (mp4a)

[–] indomara 2 points 10 months ago* (last edited 10 months ago) (1 children)

The more I read about this, the more confused I am! Near as I can tell, Mozilla refuses to support H.265 because it is "encumbered by patents". Is that right?

https://developer.mozilla.org/en-US/docs/Web/Media/Formats/Video_codecs#hevc_h.265

There are a few other places that mention the same... and I don't know of a reputable place to download codecs.

However people in this thread say they are viewing this video in Firefox just fine! o_O

Edit: I downloaded the HEVC codec from the Microsoft store and it has had no effect. (Feel no obligation to reply, at this point I am just lost!)

[–] [email protected] 2 points 10 months ago

Yeah, firefox doesnt support H.265 it looks like from some googling. Not exactly sure how other people are getting it to work, but it does look like there's some extensions for firefox to toss the media streams to VLC instead, that could work for you.