title: map.on('click') coordinates wrong in Firefox
body: <p>I am using OpenLayers 3.10.1 and the following code:</p>
  
  <pre><code>map.on('click', function (map, evt) {
      var mouseCoords = [evt.originalEvent.offsetX, evt.originalEvent.offsetY];
  
      alert(mouseCoords);
  });
  </code></pre>
  
  <p>When I click as near as I can to the upper left corner of the map in IE or Chrome, I get an alert with the mouse coordinates offset from the top left corner of the map div, i.e. something sensible like [3,4].</p>
  
  <p>But when I try the same things I get the mouse coordinates relative to the browser window and not the map div.  Does anyone know why this is?  Am I using an outdated way to find the mouse coordinates (ultimately so I can calculate which feature was clicked)?</p>