Adds MouseEvent .offsetX and .offsetY to Firefox

event-offset-polyfill.js

// polyfill for event.offsetX/offsetY
// Firefox is the only browser that doesn't support it (IE has since 4)
(function() {
  var evt = document.createEvent('MouseEvent');
  if (evt.offsetX === void 0) {
    Object.defineProperties(MouseEvent.prototype, {
      'offsetX': {
        get: function() {
          return this.layerX - this.target.offsetLeft;
        }
      }
      , 'offsetY': {
        get: function() {
          return this.layerY - this.target.offsetTop;
        }
      }
    });
  }
})();