event.offsetX in Firefox

From a JQuery bug tracker page - a nice polyfill is this:

var offX  = (e.offsetX || e.pageX - $(e.target).offset().left);

.. where e is the event returned from a jquery event. Obviously, only if you've got Jquery already on your project, otherwise will have to do the offset() stuff manually.

Unfortunately no solution worked for me.

I found a good implementation here:

var target  = e.target || e.srcElement,
              rect    = target.getBoundingClientRect(),
              offsetX = e.clientX - rect.left,
              offsetY  = e.clientY - rect.top;

e.offsetX   = offsetX;
e.offsetY   = offsetY;

Try layerX, layerY

var x = (e.offsetX === undefined) ? e.layerX : e.offsetX;
var y = (e.offsetY === undefined) ? e.layerY : e.offsetY;

FIDDLE