Well, the toplevel submenus are defined correctly, indeed, but here's one example of a deeper submenu where it's not correct:

<li><a href="..."><div style="...">Forest Management</div></a></li>
  <ul>

As you can see, it's closing the <li> tag first, and the nested list is not anymore a sublist of that item.  That's not correct HTML and DynarchMenu can have no clue that that item has a submenu.

It works in IE because IE corrects the DOM when parsing the HTML.  We have plenty of reports on this issue, it's not a bug in DynarchMenu and certainly not a bug in Firefox—the fix is easy and only requires you to write correct code. ;-)