userChrome.css

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

/* Actually compact compact mode */

#tabbrowser-tabs {
    height: 40px !important;
    
    /* Fix height when dragging tabs */
    &[movingtab] {
	padding-bottom: revert;
	margin-bottom: revert;
    }
}

/* Tab indices */

#tabbrowser-tabs {
    counter-reset: tab-index;
}

.tabbrowser-tab:nth-child(-n + 8) .tab-content::before,
.tabbrowser-tab:nth-child(n + 9):last-of-type .tab-content::before {
    counter-increment: tab-index;
    content: counter(tab-index);
    color: #555;
    font-size: 80%;
    background: #dddd;
    border-radius: 2px;
    position: absolute;
    top: 8px;
    left: 5px;
    line-height: 1;
    z-index: 1;
    pointer-events: none;
}

/* Always show close button on hover */

.tabbrowser-tab:hover .tab-close-button {
    display: revert !important;
}