Your html looks wrong to me in your second example.
The type should be closed with " not continued with ; Codecs should also use" "
I don't think the Codecs bit is needed though. Having the Type correct should be enough.
<video controls preload="none"> <source src="FooBar.mp4" type="video/webm"> </video>
Edit: Also presumably your files are definitely AV1? Double check that. I think you can also drag and drop video into Firefox to see if they will play.
Edit2: Also on searching I've seen someone say you may need to use the video tag itself for mkv:
<video controls preload="none" src="FooBar.mkv"> </video>
It's one of the solutions lower down on this stack post, but you'd need to test that. https://stackoverflow.com/questions/21192713/how-to-playback-mkv-video-in-web-browser
As a side, it's very frustrating to see how many people wrote code on that page that just works on Chrome. So much for Web standards!