As fortune has it, my Firefox looks like your IE example. That suggests that your browsers are using different fonts as their defaults. That’s a good thing because it reminds us that users may not use or have on their computer, the same fonts that the designer is using when the page is being constructed. I meander…
The attached screen shot shows Firefox with the Firebug plugin. The blue box is the outline of the ul. The pane at the bottom shows that the width is fixed at 400px.
Usually, it is better if things like the menu have a flexible or non-fixed width, or simply use the full available width of the page. In other words, delete the 400px fixed width. Is that an option for you?