Questo dovrebbe differenziare le schede attive, quelle lette e quelle non lette in base al colore:
@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
/* Tab Color Underscoring active/read/unread (DM) */
/* ===================================================== */
/* leaving background and focus rings alone so that background color
for Perma Tabs extension will prevail, but prefer Flat tabs usages */
/* Change underline color of active tab - RED */
tab[selected="true"] {
border-left: solid 1px #FF0000 !important;
border-bottom: solid 2px #FF0000 !important;
color: #000000 !important; }
/* Change underline color of normal tabs - BLUE */
tab:not([selected="true"]) {
border-left: solid 1px #0066CC !important;
border-right: solid 1px !important;
border-bottom: 2px solid #0066CC !important;
color: black !important; }
/* Change text and underline color of unread tabs - GREEN */
/* http://kb.mozillazine.org/Change_the_style_of_tab_markers */
#content tab:not([selected]) {
color: #005500 !important;
border-bottom: 2px solid #00BB00 !important;}