On page load

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Note: development documentation is in process of being moved to Mozilla Development Center (MDC).

This article is for XUL/JavaScript developers who want to have their code executed each time a new page is loaded in browser/mail.

Creating an overlay

First, you need to create an overlay to one (or more, depending on which applications you target) of the following XUL documents:

Application	URI to overlay
Firefox	chrome://browser/content/browser.xul
Thunderbird and Mozilla Mail	chrome://messenger/content/messenger.xul
Navigator from the Mozilla Suite	chrome://navigator/content/navigator.xul

Attaching a script

Attach a script to your overlay, and add a load event listener to appcontent element (browsers) or messagepane (mail) by putting this code in the script:

window.addEventListener("load", function() { myExtension.init(); }, false);

var myExtension = {
  init: function() {
    var appcontent = document.getElementById("appcontent");   // browser
    if(appcontent)
      appcontent.addEventListener("load", this.onPageLoad, true);
    var messagepane = document.getElementById("messagepane"); // mail
    if(messagepane)
      messagepane.addEventListener("load", this.onPageLoad, true);
  },

  onPageLoad: function(aEvent) {
    var doc = aEvent.originalTarget; // doc is document that triggered "onload" event
    // do something with the loaded page.
    // doc.location is a Location object (see below for a link).
    // You can use it to make your code executed on certain pages only.
    if(doc.location.href.search("forum") > -1)
      alert("a forum page is loaded");
  }
}

Current Firefox trunk nightlies will fire the onPageLoad function for not only documents, but xul:images (favicons in tabbrowser). If you only want to handle documents, ensure aEvent.originalTarget.nodeName == "#document" [1].

References

• See Chris Neale's URIid extension for a working example.
• If you need to have a more complicated listener (not just onload), use progress listeners.
• Location object