@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);

tab {
   border: #FFFFFF 1px solid; 
   -moz-border-radius-topleft: 4px !important;
   -moz-border-radius-topright: 4px !important;
   -moz-border-radius-bottomleft: 0px !important;  
   -moz-border-radius-bottomright: 0px !important; 
}