I made a copy of your web page and removed the line breaks that you were using for positioning, so I could see the problem. Then I had to look up the original menu scripting to do some comparisons. The scripting for the guest book may be setting a Z-Index but the menu does not. I believe that the original menu styling defined a Z-Index.
Below is a snippet of Javascript code from your web page. Add the highlighted element to force your dropdown menu items to be in front of the guest book. I checked it in Firefox and Internet Explorer and it seemed to work fine.