Tabs on bottom for Firefox 60, Mac OS X version (InterWeb)

userChrome.css

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

/* TABS on bottom */
#navigator-toolbox toolbar:not(#nav-bar):not(#toolbar-menubar) {
	-moz-box-ordinal-group: 10;
}
#TabsToolbar {
	-moz-box-ordinal-group: 1000 !important;
	-moz-appearance: none !important;
	background: url(chrome://browser/skin/Toolbar-background-noise.png) hsl(0,0%,83%) !important;
	border-bottom: 2px solid #b6b6b6 !important;
	height: 32px !important;
}
.tab-background-start[selected="true"]:-moz-locale-dir(ltr):not(:-moz-lwtheme)::before, .tab-background-end[selected="true"]:-moz-locale-dir(rtl):not(:-moz-lwtheme)::before,
.tab-background-end[selected="true"]:-moz-locale-dir(ltr):not(:-moz-lwtheme)::before, .tab-background-start[selected="true"]:-moz-locale-dir(rtl):not(:-moz-lwtheme)::before {
    background-size: 100% 98% !important;
}
.tab-background-middle[selected="true"] {
    background-size: var(--tab-stroke-background-size), auto 98% !important;
}