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

/* for selected tab */
tab[selected="true"] .tabs-closebutton.always {
   margin-right: -5px !important; 
   margin-top: 1px !important; 
   margin-bottom: -1px !important; }

/* for tabs not selected */
/* this works for mine, but you might need to add margin-top or margin-bottom here */
tab[selected="false"] .tabs-closebutton.always {
   margin-right: -1px !important; }