Thank you @IsoKiero@sopuli.xyz !
This is the solution.

unfortunately I can’t apply it, because the NAS is a closed proprietary 💩

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

ffmpeg -i FooBar.mkv

Stream #0:0: Video: av1 (libdav1d) (Main), yuv420p(tv, bt709, progressive)…

Thanks @Malix@sopuli.xyz ,

by any change I’ve tried codecs="avc1.4d401f" with my videos but of course it’s not working.

Whats drive me crazy, is when open trough file:/// Firefox can play it, but once inside an html page, you have to specify the right codec !? WTF

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…

deleted by creator

Thank you @taaz@biglemmowski.win no matter the type I enter, it’s still not working…

btw the videos on your link are encoded H264 with , not AV1

Codec: H264 - MPEG-4 AVC (part 10) (avc1)