Firefox Mojave Dark titlebar w/out tabs

userChrome.css

/* hide tabs in toolbar */
#main-window[tabsintitlebar="true"]:not([extradragspace="true"]) #TabsToolbar .toolbar-items {
    opacity: 0;
    pointer-events: none;
}
#main-window:not([tabsintitlebar="true"]) #TabsToolbar {
    visibility: collapse !important;
}
/* clean up borders and color without tabs*/
#navigator-toolbox {
    --tabs-border-color: none !important;
}
#main-window[tabsintitlebar="true"]:not([extradragspace="true"]) #TabsToolbar {
    background-color: rgb(50,50,52);
    border-color: rgb(50,50,52);
    height: 25px;
}
#TabsToolbar .titlebar-spacer {
    border-inline-end: none !important;
}