cdf4825d_ompatible_with_Mozilla_Firefox_vent_object_properties_methods_Description
[In Explorer] offsetX, offsetY [In Firefox (DOM standard)] If the mouse is on a absolutely, fixed or relatively positioned element, then you can use layerX, layerY (non-standard). However, event.target is in a normally (static) positioned element these properties will give you the offset with respect to the document root element (which normally corresponds to the page). In this case your only option is to manually calculate the "offsetX" value by traversing all the hierarchy from the document root element, adding the objects' offsetLeft / offsetTop values. [Description] 
Position of the event with respect to the element that generates it.
Making your web page compatible with Mozilla Firefox
Description