event.offsetX in Firefox

Answer 1

There's a bug in Musa's solution: think what happens if e.offsetX === 0 and e.layerX === undefined...

var x = e.offsetX || e.layerX; // x is now undefined!

A more robust version is as follows:

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

Or, because we can assume that if offsetX is defined, offsetY will be too:

var hasOffset = e.hasOwnProperty('offsetX'),
    x         = hasOffset ? e.offsetX : e.layerX,
    y         = hasOffset ? e.offsetY : e.layerY;