title: Firefox Extension Menuitem Image Doesn't Display
body: <p>I'm working on a Firefox extension that creates a menu of bookmarks from data fetched from a server. I'd like to add icons to each menuitem, but I can't seem to get them to display. I'm creating each menuitem using:</p>
  
  <pre class="lang-js prettyprint-override"><code>Bookmarks.createMenuItem = function(item) {
      var menuItem = document.createElementNS(Bookmarks.XUL_NS, "menuitem");
      menuItem.setAttribute("label", item.url_title);
      menuItem.setAttribute("oncommand", "Bookmarks.openUrl('" + item.url + "');");
      menuItem.setAttribute("class", "bookmark-menu-item");
      if ( item.favicon ) {
              menuItem.setAttribute("class", menuItem.getAttribute("class") +
                                     " menuitem-iconic");
              menuItem.setAttribute("image", item.favicon);
      }
  
      return menuItem;
  };
  </code></pre>
  
  <p>The menuitem works just fine except that it doesn't display the image. I've verified the URLs that I'm getting for each image (from item.favicon in the code below) are accessible.</p>
  
  <p>Any idea what's wrong here? I'm using Firefox 6.0.2.</p>