Sorry, there was a problem with the stylesheet. The div that holds the logo and banner ad in the header area (div#sioptions) was too tall, and overlapped the menu. This prevented Firefox from recognizing the mouse hovering over the menu items. Apparently browsers interpreted this overlap differently. Also, this problem only came up when the custom code box wasn't used. Anyway, I removed that div's height property (don't think it's needed really), so the problem should be solved.
The Kubrick package has the updated file, or you can get it at this link: kubrick.css (or just remove height: 176px; at about line 273 of your kubrick.css).