I can't imagine why Mozilla hasn't fixed the menu bug yet. And, I can't believe no one else is bothered by it.
Doesn't anyone use full-screen mode? Anytime you switch Firefox to full screen (F11) and try to access a menu using an Alt key combination--for example, Alt-F to bring down the file menu--not only does the menu not come down, when you exit full-screen mode you won't be able to bring that menu down again in that window until you restart Firefox.
Perhaps the whole world is so addicted to their mice that they haven't noticed, having almost completely abandoned the keyboard.