I've found that the RC is not working in Mozilla. I'm only guessing this is a bug and hope I've just not gotten something wrong.
Anyway, I've got an example of both the beta version 4.13 and RC5 up here.
http://dev.benzot.net/v4/
and
http://dev.benzot.net/v5/
It's the sub menu that is not working. Take a look.