Firefox extension: run when page loaded

I'm trying to write firefox extension which will run when a (specific) page is loaded (it will be same key words replace).

I write code like:

window.addEventListener("load", function()  maApp.init(); }, false);
var maApp= {

  init: function() {
    var appcontent = document.getElementById("appcontent");   // browser
    if(appcontent)
      appcontent.addEventListener("DOMContentLoaded", maApp.onPageLoad, true);
    var messagepane = document.getElementById("messagepane"); // mail
    if(messagepane)
      messagepane.addEventListener("load", function(event) { maApp.onPageLoad(event); }, true);
  },
    onPageLoad: function() {
           alert("test);
        doSomething();
    }                       
};

But onPageLoad is never run.开发者_如何学Go.. no alert... Can somebody please tell me what I'm doing wrong?

---

First some words on getting the browser element. In Firefox this element has ID content, not appcontent. Still, the recommended way of getting it is the window.gBrowser variable. In Thunderbird 5 the ID of the browser element changed so your code will stop working - rather than going by ID you should use window.messageContent variable which will work both in the current and future versions. Together you get:

var browser = null;
if ("gBrowser" in window)
  browser = window.gBrowser;             // Firefox and SeaMonkey Browser
else if ("messageContent" in window)
  browser = window.messageContent;       // Thunderbird
else if ("getMessageBrowser" in window)
  browser = window.getMessageBrowser();  // SeaMonkey Mail

if (browser)
  ...

Now about listening to page loads, the recommended approach here is progress listeners - see https://developer.mozilla.org/en/Code_snippets/Progress_Listeners. You attach a progress listener to the browser and look for state changes:

onStateChange: function(aWebProgress, aRequest, aFlag, aStatus)
{
  if ((aFlag & Components.interfaces.nsIWebProgressListener.STATE_STOP) &&
      (aFlag & Components.interfaces.nsIWebProgressListener.STATE_IS_WINDOW))
  {
    // A window finished loading
    doSomething(aWebProgress.DOMWindow);
  }
}

---

The accepted answer is outdated. The solution using the WebExtensions API would be

browser.tabs.onUpdated.addListener(function(tabId, changeInfo) {
    if (changeInfo.status == "complete") {
        //add your script
    }
})

browser.tabs.onUpdated.addListener Listens to events within the Tab (Reference)

(changeInfo.status == "complete") filters the right event and executes the commands within the if statement.