Event Handler For Onfocus Fails In Firefox?

I am successfully using this code to cause an event to fire when the user clicks inside any of the text inputs in my form:

Code:

govindaAddEventHandler(document.getElementById("input_" + arrWhichFieldsFilterable[i]),"click",getMouseCoords); // govindaAddEventHandler(obj, eventName, handler)

...which relies on this:

Code:

function govindaAddEventHandler(obj, eventName, handler) {
if (document.attachEvent) {
obj.attachEvent("on" + eventName, handler);

[code]....

The problem occurs when I change it from an on"click" event to an on"focus" event. In the latter case (onfocus) it does not work on Firefox, but only in IE8. Why is that? How can I add an event handler for onfocus in firefox?