The click handler is registered on the canvas and it checks e.offsetX. On Firefox, some people use e.layerX, but it's not exactly the same thing [0]. Someone posted a useful polyfill on JQuery's bugtracker [1].

Firefox should just support offsetX as well, but they don't, for whatever reason.

[0] https://bugzilla.mozilla.org/show_bug.cgi?id=122665#c3

[1] http://bugs.jquery.com/ticket/8523#comment:16