title: NoModificationAllowedError in Firefox when using DnD events
body: <p>Using some basic drag'n'drop features and i keep getting this message in Firefox, works well in Chrome though.</p>
  
  <p><code>NoModificationAllowedError: Modifications are not allowed for this document</code></p>
  
  <p>This happens when i try to use the <code>clearData()</code> function on the event when the <code>ondrop</code> event is fired.</p>
  
  <p>HTML:</p>
  
  <pre><code>&lt;!-- draggable element --&gt;
  &lt;div draggable="true" ondragstart="onDragStart(event);"&gt;
  &lt;!-- dropzone element --&gt;
  &lt;div ondragover="onDragOver(event);" ondragenter="onDragEnter(event);" ondrop="onDrop(event);"&gt;&lt;/div&gt;
  </code></pre>
  
  <p>JS:</p>
  
  <pre><code>function onDragStart(event) {
      event
          .dataTransfer
          .setData('text/plain', 'test');
  }
  function onDragOver(event) {
      event.preventDefault();
  }
  function onDragEnter(event) {
      event.preventDefault(); // polyfill fix
  }
  function onDrop(event) {
      event.preventDefault();
      event
          .currentTarget
          .parentNode
          .classList.add("dropped");
      event
          .dataTransfer
          .clearData(); //&lt;---- This is where it fails
  }
  </code></pre>
  
  <p>So yeah no idea how to fix this, and i need to clear the data as i set/reset some variable and internal params using it (code omitted). Works well in chrome (even IE) </p>