OK, I made work the mp4 container

So in all my trial , I have introduce en error in the video path /path/path/FooBar.mp4 as those test where taking place under Windows this time, the front / was making the file unreachable, “funny” in the console no error about that, but about codec that is not found… O_o

MP4

it worked with

...
<source src="path/FooBar.mp4" type="video/mp4">
...
<!-- so without specifying the codec, at least -->

MKV

I’ve tried everything so far, I didn’t managed to make it work :/ so it seem that finally Firefox do not support MKV with AV1

btw:

when I open .mp4 container version of it, by file path (url bar) like file:///X:/path/FooBar.mp4 it works, so Firefox can Open it… (now still to make it work inside an html page )

for .mkv it trigger a file download…