Programmatically typing into a textbox (KeyboardEvent)

Hello,

I want to programmatically type into a xul textbox (xulrunner 2.0). 
While the following code does trigger the "not canceled"-alert, it 
doesn't change the contents of the textbox:

XUL:
-------------------------------------

   <textbox id="testbox"/>

JS (triggered on button command);
-------------------------------------

   var test_el = document.getElementById("testbox");
   var evt = document.createEvent("KeyboardEvent");
   evt.initKeyEvent("keypress", true, true, null, false, false, false, 
false, 65, 0); //65 is character 'a'

   var canceled = !test_el.dispatchEvent(evt);
   if(canceled) {
     // A handler called preventDefault
     alert("canceled");
   } else {
     // None of the handlers called preventDefault
     alert("not canceled");
   }


A similar example with MouseEvents (click) triggering a checkbox works 
fine for me, but I can't get the KeyboardEvent working (entering text in 
the textbox). Any help is greatly appreciated.

regards,
Michael