simple normalization of jquery event so offsetX / offsetY can be used cross browsers

lolrus.js

/**
 * normalizeEvent
 *
 * Firefox does not implement offsetX, OffsetY, so we have to detect for this an
 * manually calculate it ourselves using the pageX, pageY less the event
 * target's left offset and right offset
 *
 * If using a browser that supports offsetX, OffsetY, just return the event,
 * don't need to do anything
 */
var normalizeEvent = function(event) {
  if(!event.offsetX) {
    event.offsetX = (event.pageX - $(event.target).offset().left);
    event.offsetY = (event.pageY - $(event.target).offset().top);
  }
  return event;
};

LOLrus.prototype.someMethodThatDependsOnEventPosition = function() {
  $(this._someIternalHtmlDomEl)
    .unbind('click')
    .mousedown(_.bind(function(e) {
      e = normalizeEvent(e);
      this._somethingThatWantsCurrentMouseDownPosition(e.offsetX, e.offsetY);
    }, this);
}

Nice polyfill! If you want to be definitely insane and possibly awesome, you can overwrite mousedown to do this for you:

(function() {
  var originalFilter = $.event.fixHooks.mousedown.filter;

  var funkytown = function(event, original) {
    event = originalFilter(event, original);

    if(!event.offsetX) {
      event.offsetX = (event.pageX - $(event.target).offset().left);
      event.offsetY = (event.pageY - $(event.target).offset().top);
    }

    return event;
  }

  jQuery.event.fixHooks.mousedown.filter = funkytown;
}());

Of course, if you ship this code there is a 90% chance you will be struck by a car, and simultaneously by lightning, next time you cross the street.

firefox have layerXand layerY value;

div.on('click',function(e) {
    var x = (e.offsetX != null) ? e.offsetX : e.originalEvent.layerX;
    var y = (e.offsetY != null) ? e.offsetY : e.originalEvent.layerY;
});

(event.pageX - $(event.target).offset().left); method dont works with transformed elements (css3 rotate(), scale(), translate() ...)