XUL DOMContentLoaded - can't find anything about the propertoes of the returned event

Hello all,

I'm trying create a XUL plugin to get the HTTP status of a page.   I've 
found that I could use "window.addEventListener" with the "DOMContentLoaded" 
argument.  The callback works, as I could see using "console.log( )"

The problem is that I cannot find much of anything about the "event" object 
the callback provides. :-(

"event.type" returns "DOMContentLoaded" (the event I selected, duh.), but 
have not been able to find more.  "event.status" and "event.readyState" 
(found on the web somewhere) both return "undefined".

Does anyone know where to find more about that "event" objects 
properties/methods - and/or tell me why the "event.status" is undefined ?

Maybe even better : example code showing how to extract the HTTP status code 
from that "event" object ...

The involved (minimal) code is the below:
- - - - - - - - - - - - - - - - - -
window.addEventListener('DOMContentLoaded', EventDomLoaded, false);

function EventDomLoaded(event) {
  console.log('EventDomLoaded: ' + event.type +" "+ event.status +" "+ 
event.readyState);
}
- - - - - - - - - - - - - - - - - -

Firefox 52.5.0 ESR

Regards,
Rudy Wieser