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

/* Fade the entire tab (icon, text and frame)
#content tab:not([selected="true"]) 
  {-moz-opacity: 0.5 !important;}
*/

/* Fade just the icon and the text */
#content tab:not([selected="true"]) .tab-middle
  {-moz-opacity: 0.5 !important;}