function getLastChild(aNode) {
  if (!PlacesUtils.nodeIsFolder(aNode) &&
      !PlacesUtils.nodeIsQuery(aNode))
      return aNode;
  aNode = aNode.QueryInterface(Ci.nsINavHistoryContainerResultNode);
  aNode.containerOpen = true;
  return arguments.callee(aNode.getChild(aNode.childCount-1));
}
var result = HistoryService.executeQuery(query, options);
getLastChild(result.root);