firefox extension run on each page

I am new to firefox extension development. I am building an extension which needs to run for each page the user browses. At the moment the extension runs when firefox window starts. I have pasted code below.

XUL Code:

<?xml version="1.0"?>
<?xml-stylesheet href="chrome://global/skin/" type="text/css"?>
<window id="mainWin" title="my extension" xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
<script type="application/x-javascript" src="chrome://myextension/content/myextension.js" />
</window>

and in the myextension.js Javascript file I have an alert(window.location.href);

I want to开发者_运维知识库 run this extension and get the url of the page I am browsing each time.

I might be going in a wrong direction. Any help much appreciated.

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Simpler: https://developer.mozilla.org/en/Code_snippets/On_page_load

More control: https://developer.mozilla.org/en/Code_snippets/Progress_Listeners

Both linked from https://developer.mozilla.org/en/Code_snippets, also see https://developer.mozilla.org/en/Extensions and https://developer.mozilla.org/en/Extensions/Firefox.