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

.tabbrowser-tab[selected="true"] {
  background-color: DodgerBlue !important;
  opacity: 1 !important; }

.tabbrowser-tab:not([selected="true"]) {
  opacity: 0.5 !important; }

.tabbrowser-tab:not([selected="true"]):hover {
  background-color: LightSkyBlue !important; 
  opacity: 0.5 !important; }