event.offsetX in Firefox

Answer 1

offset actually doesn't translate directly into layer; the offset property doesn't account for the element's margin. The code below should account for this.

function(e) {
    var x = e.offsetX, y = e.offsetY;
    if(e.hasOwnProperty('layerX')) {
      x = e.layerX - e.currentTarget.offsetLeft;
      y = e.layerY - e.currentTarget.offsetTop;
    }
}