Pop direction parameter?

We are using a horizontal menu to display Primary Links. You can see the site at http://wsbe2.unh.edu (not yet "published," but available for review). We have noticed that when we view the site on IE, menus always drop down (the desired behavior). However, on Firefox, the menus will drop up, instead, unless there wouldn't be room to display, in which case they drop down. Is there a parameter to force the direction of the menu expansion? I've looked through the documentation and examples, and I don't see mention of one, but thought I would check. Thanks!