@media not (-moz-bool-pref: "theme.nosidebarscrollbar.before180b") {
    #zen-tabs-wrapper {
        scrollbar-width: none !important;
    }
}

@media (-moz-bool-pref: "theme.nosidebarscrollbar.before180b") {
    #zen-browser-tabs-wrapper {
        scrollbar-width: none !important;
    }
}