FireFox Extension XulSchool - Adding Events and Commands - on load function - firefox-addon

Question

I'm trying to edit the helloworld from this tutorial:
https://developer.mozilla.org/en-US/Add-ons/Overlay_Extensions/XUL_School/Getting_Started_with_Firefox_Extensions
So, I need to execute a function when page loads, this part of the tutorial explains how to do that:
https://developer.mozilla.org/en-US/Add-ons/Overlay_Extensions/XUL_School/Adding_Events_and_Commands
I edited the browserOverlay.js file, I writed the init function and that listener at the end of the code:
/**
* XULSchoolChrome namespace.
*/
if ("undefined" == typeof(XULSchoolChrome)) {
var XULSchoolChrome = {};
};
/**
* Controls the browser overlay for the Hello World extension.
*/
XULSchoolChrome.BrowserOverlay = {
init : function(event) {
window.alert('aaaaeeee');
},
/**
* Says 'Hello' to the user.
*/
sayHello : function(aEvent) {
let stringBundle = document.getElementById("xulschoolhello-string-bundle");
let message = stringBundle.getString("xulschoolhello.greeting.label");
window.alert(message);
}
};
window.addEventListener("load", function() {
//window.removeEventListener("load", onFirefoxLoadEvent, false); // remove listener, no longer needed
XULSchoolChrome.BrowserOverlay.init();
}, false);
and I edited the broserOverlay.xul, I writed the onload propertie at overlay tag:
<?xml version="1.0"?>
<?xml-stylesheet type="text/css" href="chrome://global/skin/" ?>
<?xml-stylesheet type="text/css"
href="chrome://xulschoolhello/skin/browserOverlay.css" ?>
<!DOCTYPE overlay SYSTEM
"chrome://xulschoolhello/locale/browserOverlay.dtd">
<overlay id="xulschoolhello-browser-overlay"
onload="XulSchoolChrome.BrowserOverlay.init();"
xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
<script type="application/x-javascript"
src="chrome://xulschoolhello/content/browserOverlay.js" />
<stringbundleset id="stringbundleset">
<stringbundle id="xulschoolhello-string-bundle"
src="chrome://xulschoolhello/locale/browserOverlay.properties" />
</stringbundleset>
<menubar id="main-menubar">
<menu id="xulschoolhello-hello-menu" label="&xulschoolhello.hello.label;"
accesskey="&xulschoolhello.helloMenu.accesskey;" insertafter="helpMenu">
<menupopup>
<menuitem id="xulschoolhello-hello-menu-item"
label="&xulschoolhello.hello.label;"
accesskey="&xulschoolhello.helloItem.accesskey;"
oncommand="XULSchoolChrome.BrowserOverlay.sayHello(event);" />
</menupopup>
</menu>
</menubar>
<vbox id="appmenuSecondaryPane">
<menu id="xulschoolhello-hello-menu-2" label="&xulschoolhello.hello.label;"
accesskey="&xulschoolhello.helloMenu.accesskey;"
insertafter="appmenu_addons">
<menupopup>
<menuitem id="xulschoolhello-hello-menu-item-2"
label="&xulschoolhello.hello.label;"
accesskey="&xulschoolhello.helloItem.accesskey;"
oncommand="XULSchoolChrome.BrowserOverlay.sayHello(event);" />
</menupopup>
</menu>
</vbox>
</overlay>
It does not alert on page loads.... why?