userChrome.css

/*
 * Do not remove the @namespace line -- it's required for correct functioning
 */
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */
/*
 * Hide tab bar, navigation bar and scrollbars
 * !important may be added to force override, but not necessary
 * #content is not necessary to hide scroll bars
 */
#TabsToolbar {visibility: collapse;}
#navigator-toolbox {visibility: collapse;}
#PersonalToolbar { display:collapse!important; }
#CustomizeToolbarWindow { display:collapse!important; }
  }

  /* Theme - Compatibility ****************************************************/
  /* Header Image */
  :root[lwtheme-image] {
      background-image: var(--lwt-header-image)  !important; /* Original: var(--lwt-header-image) */
      background-repeat: no-repeat !important;
      background-position: right top !important;
  }
  :root[lwtheme-image] #navigator-toolbox:-moz-lwtheme {
      background-image:    var(--lwt-additional-images) !important; /* Original: var(--lwt-header-image), var(--lwt-additional-images); */
      background-repeat:   var(--lwt-background-tiling) !important;
      background-position: var(--lwt-background-alignment) !important;
      background-color:    unset !important; /* Original:  var(--lwt-accent-color) */
  }