The problem is within "igmenu_mouseup" method.

There is a reference to "this.__igmenu_navigateUrl",  IE sees this as being associated to "window" since its a standalone function and not tied to an instance of an object.

 Here is a real easy fix, just include this at the bottom of the page before the "</form>" tag.  I put all mine in a "fix-mozilla.js" and a "fix-ie.js" file.

_igmenu_mouseup = igmenu_mouseup;
igmenu_mouseup = function(){
  var args = [;
  for (var i=0, len = arguments.length; i < len; i++)
    args[i] = arguments[i];
  return _igmenu_mouseup.apply(window, args);
}

 There is also a problem wiht the "igmenu_keyboardWith508Support.onmenufocus" method as well and a few more.

 User FireBug to see the errors and the js methods where they occur.