Display in IE vs Firefox: IE properly shows Nav menu, Firefox does not

I just downloaded the most recent version of WP 3.1. Then I purchased the most recent Thesis Theme 1.8, and installed that over WP 3.1. Then I purchased the most recent version of the Folio skin for thesis and installed that. The only thing I have changed was the background color at the top, just under the Nav menu, and it’s a pretty standard change, with a drop down menu from within the skin’s admin menu.

In IE, my Nav menu displays and functions properly in that when I click on a tab in my Nav menu, the correct page displays.

In Firefox, my Nav menu wraps around itself and when I click on it it takes me to my RSS feed page, every single time.

It seems like the page would show up the same way in both browsers, seeing as I have the most recent version of WP and Thesis and my Folio skin and I get Firefox updates all the time, and I haven’t yet touched the code.

Does anyone have any insight as to why this is happening and what I can do to fix it? Matt Hodder (Folio skin developer) says its probably a WP issue.

Here is my site: https://www.cepilz.com

Thanks.