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

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

MP4 container part, solved --> https://lemmy.ml/post/22627660

Funny 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…

Thank you BananaTrifleViolin,

I’ve tried all your solutions, none works.

Yes they are AV1 (I have encoded them)

Codec: AOMedia’s AV1 Video (av01)
Color primaries: ITU-R BT.709
(audio): Codec: MPEG AAC Audio (mp4a)\

\

… So much for Web standards! …

Couldn’t agree more

I think you can also drag and drop video into Firefox to see if they will play.

no, this initiated a download :/