The reason it's dropping (which it is here in windows) is simply that the total width of all menu items exceeds the 720px width of the UL. Solution? Make the UL wider... BUT:
That fixed width layout with fixed metric (px) fonts is an accessibility mess, and really the menu and elements around it SHOULD be designed to go narrower than one single width. Between that and the massive code bloat, and design elements that to be frank have no business on a website if you care about users getting to it, results in a page that -- as I often tell people -- I'd throw away and start over from scratch, as from an accessibility, usability, functionality, and efficiency standpoint there is little if anything I'd try to salvage from that.
Sorry if that sounds harsh, but it's the truth of the matter; there's more to building a proper website than what it looks like on the screen you happen to be seated in front of.