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 )