[Greasemonkey] addEventListener, DOMNodeInserted event in GM user script

[Brian Donovan]

Given:

document.addEventListener('DOMNodeInserted',function(){alert('DOMNodeInserted
!');},true);
window.addEventListener('load',function(){alert('load !');},true);

Why is it that both of the following alerts will pop up in Firefox
when the following lines are placed within a script tag within a page
but only the latter (for the window.onload event) will pop when those
lines are present in a GM user script?

Thanks,
Briandon