Comment by dredmorbius

    $ cat userChrome.css 
    /* Mozilla chrome (UI) styling */

    /* Hide tabs bar
     *
     * Fri Nov 17 12:23:47 CST 2017
     * https://superuser.com/questions/1268732/how-to-hide-tab-bar-tabstrip-in-firefox-57-quantum
     * https://www.ghacks.net/2017/09/27/tree-style-tab-is-a-webextension-now/
     */

    #tabbrowser-tabs { visibility: collapse !important; }