Problems detecting Opera browser

I'm trying to help someone with this menu (which is part of a frameset):
http://www.kepler.fr.bw.schule.de/menue.html

The menu shows up fine in IE and FF, but not in Opera. Latest version is
9.1, and apparently some earlier versions have not had the problem with not
showing the menu.

Could you guys please take a look at the javascript of the file and try to
help me with the browser detection settings?

Much appreciated,
Inger