Use of mouse event offsetX and offsetY in Firefox. With jQuery 1.7.2

gistfile1.js

$("#element").click(function (ev) {

  if(typeof ev.offsetX === "undefined" || typeof ev.offsetY === "undefined") {
    var targetOffset = $(ev.target).offset();
    ev.offsetX = ev.pageX - targetOffset.left;
    ev.offsetY = ev.pageY - targetOffset.top;
  }    

  // now ev.offsetX and ev.offsetY can be used
  // ...

});