numbering on each tab for Firefox

tabnumber.css

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

#TabsToolbar {
    counter-reset: tabnumber;
}

#TabsToolbar tab {
    counter-increment: tabnumber;
}

#TabsToolbar tab:not([pinned])::before {
    display: inline-block;
    position: relative;
    z-index:65535;

    width: 4em;
    margin-right: -4em;
    padding-top: 10px;

    content: counter(tabnumber);
    font-family: menlo, monospace;
    font-size: 0.8em;    
    font-weight: bold;
    color: #eee;
    text-shadow: rgba(0,0,0,.2) 1px 0,
                 rgba(0,0,0,.2) 0 1px,
                 rgba(0,0,0,.2) -1px 0,
                 rgba(0,0,0,.2) 0 -1px,
                 blue 0 0 0.4em;
}