target.on_DOMMenuItemActive = function(event) {
  /*
  if (super.on_DOMMenuItemActive) {
    super.on_DOMMenuItemActive(event);
  }
  */
  let elt = event.target;
  if (elt.parentNode != this) {
    return;
  }

  if (window.XULBrowserWindow) {
    let placesNode = elt._placesNode;

    var linkURI;
    if (placesNode && PlacesUtils.nodeIsURI(placesNode)) {
      linkURI = placesNode.uri;
    } else if (elt.hasAttribute("targetURI")) {
      linkURI = elt.getAttribute("targetURI");
    }

    if (linkURI) {
      window.XULBrowserWindow.setOverLink(linkURI);
    }
  }
}.bind(target);

を

const shadow = target.shadowRoot;

の前に追加してみるとか