I made the video by encoding some random clip with ffmpeg -i random_video.mp4 -c:v libaom-av1 -crf 30 test.mp4 (seems to work just as well with libsvtav1)
and you're absolutely sure the files are av1? if you try to open the video files with ffmpeg, eg: ffmpeg -i videofile.mkv, what does it say the codec is?
at least my av1 videos say: Stream #0:0[0x1](und): Video: av1 (libdav1d) (Main) (av01 / 0x31307661), yuv420p(tv, progressive), 854x854, 464 kb/s, SAR 1:1 DAR 1:1, 30 fps, 30 tbr, 16k tbn (default)
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
I know, I've linked that to show the correct mimetype.
Is your web server returning the correct mimetype for the FooBar.mp4 file ? Also codecs part should probably look more like this: type='video/mp4; codecs="av1, ogg"' (omit the ogg in your case)
E: if you are opening the html with the <video /> element as a file:// in firefox, you probably wont be able to load resources from http:// (only https:// it seems), and it will show the same error in this case but it's because of security not because it can't decode the media. You can see the detailed error (for loading the file) in developer tools ctrl+shift+i and switch to network tab.
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 )