Does anyone know?
It would cool 8) if there was.
I would use it 
I don’t think so… Maybe the graphics guys get excited with that ;D
FWIW, I’m not too sure that I would use an Avast persona theme on FF4, mainly because persona themes are interactive with Mozilla servers and (to my way of thinking) thereby vulnerable to injected code attacks.
@the OP: from what I read persona skins are not too difficult to develop: they’re very light (as opposed to a FF full custom skin) and there must be some sort of kit to make your own.
Anyway here’s the tuto:
http://www.getpersonas.com/en-US/demo_create