Apparently it doesn't work! Is there an export that would help you? Like Mac or Windows? I posted the Windows one as well, but wasn't sure how effective the Mac ones are (from my non-Mac machine). It will probably work, like everything Godot except these darn web builds! I did learn this is supposed to fix the FF issue for good: https://candordev.itch.io/foxfix
But I neither endorse nor do I know anything about it.