Superfish - Bordering sublevels

Superfish - Bordering sublevels

http://justtesting.atspace.com/index1.htm

Compare the page in Firefox vs Internet Explorer.
When in Explorer, the underlying sublevels are bordered black all around the drop down box, just like it should be.
However, when in Firefox the upper right and lower left parts are not bordered.

How to solve this problem when using Firefox?