title: Async ajax upload firefox doesn't work
body: <p>My ajax async works fine in Chrome, but not in Firefox. How to adapt it in Firefox ? </p>
  
  <pre><code>html2canvas($('#col2Game'), {
      onrendered: function (canvas) {
          $.ajax({
              type: "POST",
              async: false,
              url: "upload.php?id="+idPicture,
              data: { 'base64data' : canvas.toDataURL() },
          });
      }
  });
  </code></pre>
  
  <p>This is to upload pictures.</p>
  
  <p>This is the error when I get the ajax call :</p>
  
  <p><img src="https://i.stack.imgur.com/qpx9a.png" alt="enter image description here"></p>