Append download link to instagram photos (userscript)

InstagramImgDownload.js

// ==UserScript==
// @name         Instagram image in a new tab
// @namespace    http://shvalyov.ru
// @version      0.1
// @description  append download link to photos
// @author       dshster
// @match        https://www.instagram.com/*
// @grant        none
// ==/UserScript==

(function(window, undefined) {
  'use strict';
  
  var document = window.document;  
  var $main = document.querySelector('main > section > div > div');
  
  function appendDownloadLinkToArticle(articleNode) {
    var downloadLink = document.createElement('a');
    var header = articleNode.querySelector('header');
    var image = articleNode.querySelector('div > div > div > img[src]');
    var imageUrl = image.src;
    var filename = imageUrl.split('/').pop();
    
    downloadLink.href = imageUrl;
    downloadLink.setAttribute('download', filename);
    
    downloadLink.appendChild(document.createTextNode('Download image'));
    header.appendChild(downloadLink);
    downloadLink.onclick = function(event) {
      
      event.preventDefault();
    }
  }

  var observer = new window.MutationObserver(function(mutations) {
     mutations.forEach(function(mutation) {
       var article;
       if (mutation.addedNodes.length === 1) {
         article = mutation.addedNodes[0];
         
         if (article.nodeName === 'ARTICLE') {
           appendDownloadLinkToArticle(article);
         }
       }
     });
  });
  
  observer.observe($main, { childList: true });
  
  var articles = document.querySelectorAll('article');
  Array.prototype.forEach.call(articles, function(article) {
    appendDownloadLinkToArticle(article);
  });
})(window);

Firefox 84.0.1 x64, Tampermonkey 4.11.6120 - не распознается, как userscript.