deactivate tabs bar in Firefox 57+, see https://bugzilla.mozilla.org/show_bug.cgi?id=1332447#c42 and https://lifehacker.com/197715/customize-firefox-with-userchromecss

userChrome.css

@-moz-document url("chrome://browser/content/browser.xhtml") {
  #toolbar-menubar {
    display: none;
  }

  #TabsToolbar {
    height: 0px;
  }

  #tabbrowser-tabs {
    visibility: collapse;
  }
}