Nice Menu can't display properly in Firefox

Barlow is really a beautiful theme and it's always one of my favorite. There's just a small annoying problem for me. I am using Barlow theme and I've installed Nice Menus module (Head version) on my Drupal 5, and enabled the Nice Menu Block on the top of left sidebar.

The problem is when I use IE to view my website and point at the Nice Menu, it works properly as drop-dwon the menu to right and extends over the main content area. But when I use Firefox to browse it, the nice menu still can drop-down to right but kinda get "blocked" by main content area, so I barely can see the nice menu extended from left sidebar. I've used many other themes to test (like garland, bluemarine...etc) and they all works fine for Firefox. Because I'm new for css, so just wondering is there any way I can do to fix this little problem. Hope this won't cause too much inconveinece for you. Thanks in advance!