Firefox iframe and overflow div flaw?

In the page http://greywolfdesign.com/tmp/suckerfish.htm, move the mouse
point to the horizontal menu bar will pull down sub menus cover a div, a div
with overflow and a iframe. However, mouse cannot move to the pulldown menu
on top of iframe and overflow div.

Steps to Reproduce:
1. Go to http://greywolfdesign.com/tmp/suckerfish.htm
2. move mouse to the "Species" or "Links" menu item.
3. move mouse down the pull down menu.

However, IE works fine

Anyone know how to workaround this problem? (Except the one hiding the
iframe div.)