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The accepted answer is outdated.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.