I think that only the file extension is changed, so you can try to change the 3gp file extension to mp4 to see whether that gives a working file.

• /questions/1357736 No Video with supported format and MIME type found

See also this bug report

• 1749294 - File with file name ending in .mp4 has the .mp4 portion changed to .3gp when downloading

(please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html)

This doesn't answer the question/solve the problem - which is why does this happen when downloading the files in Firefox - but not in Chrome??

I think this is a mixture of server support that doesn't specify a file name via the content-disposition header and Firefox doing content sniffing that makes Firefox think this is a 3gpp file although this extension is normally aimed at 3G support for mobile devices. I don't know if you have read the comment in the bug report I linked above as that includes some comments from the devs.

You can possible check HTTP response headers in the inspector to see how the server sends those MP4 files.

Can you post a link to a publicly accessible page (i.e. no authentication or signing on required) with a link to such a video ?

Yes, I attempted to read the bug report but found it very confusing / did not find a usable answer. What I gathered from the bug report is that Mozilla isn't interested in fixing the issue and is forcing me to switch to Chrome. If Chrome can figure it out - why can't Mozilla?????

It's a massive waste of time to always switch over to Chrome in order to work with my mp4 files. I cannot waste more time trying to fix the issue so I can stay with Firefox and work with my Flickr mp4.