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

.tabbrowser-tabs { margin-bottom: -20px !important; }
#content > tabbox .tabbrowser-strip { margin-top: -1px !important; }
.tabbrowser-tabs:hover { margin-bottom: 0px !important; }
#content > tabbox .tabbrowser-strip:hover { margin-bottom: 0px !important; }