>>933
.tab-background-start[selected="true"]:-moz-locale-dir(ltr):not(:-moz-lwtheme)::before{
background-image: url(chrome://browser/skin/tabbrowser/tab-selected-start.svg);
}

.tab-background-middle[selected="true"] {
background-color: -moz-dialog;
background-image: url(chrome://browser/skin/tabbrowser/tab-active-middle.png), linear-gradient(transparent 2px, rgba(255,255,255,.4) 2px, rgba(255,255,255,.4)), none;
}

.tab-background-end[selected="true"]:-moz-locale-dir(ltr):not(:-moz-lwtheme)::before {
background-image: url(chrome://browser/skin/tabbrowser/tab-selected-end.svg);
}

このあたりを弄れ