Pls check http://uniquorn.com/dnn/  in Firefox or Chrome

If I point to the submenus of the menu "About Gregg", the submenus have transparent background in FF and Chrome except the active menu, why is this?

My CSS for the submenu seems ok :

.MainMenu_SubMenu {
    z-index: 1000;
    cursor: pointer;
 color: #ffe300;
     font-family: Verdana;
   
background-color: #7b2203;
height: 20px;
}

can any body help figure this issue?

Thanks I tried installing Firebug but after installation it just opens up my Thunderbird and nothing happens after that.. How do you use Firebug?