Add-ons using the techniques described in this document are considered a legacy technology in Firefox. Don't use these techniques to develop new add-ons. Use WebExtensions instead. If you maintain an add-on which uses the techniques described here, consider migrating it to use WebExtensions.

From Firefox 53 onwards, no new legacy add-ons will be accepted on (AMO).

From Firefox 57 onwards, WebExtensions will be the only supported extension type, and Firefox will not load other types.

Even before Firefox 57, changes coming up in the Firefox platform will break many legacy extensions. These changes include multiprocess Firefox (e10s), sandboxing, and multiple content processes. Legacy extensions that are affected by these changes should migrate to WebExtensions if they can. See the "Compatibility Milestones" document for more.

A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.

Although you can now natively parse HTML using DOMParser and XMLHttpRequest, this is a new feature that is not yet supported by all browsers in use in the wild. The code snippets on this page will let your site work until these new features are more widely available.

Safely parsing simple HTML to DOM 

When using XMLHttpRequest to get the HTML of a remote webpage, it is often advantageous to turn that HTML string into DOM for easier manipulation. However, there are potential dangers involved in injecting remote content in a privileged context in your extension, so it can be desirable to parse the HTML safely.

The function below will safely parse simple HTML and return a DOM object which can be manipulated like web page elements. This will remove tags like <script>, <style>, <head>, <body>, <title>, and <iframe>. It will also remove all JavaScript, including element attributes that contain JavaScript.

function HTMLParser(aHTMLString){
  var html = document.implementation.createDocument("", "html", null),
    body = document.createElementNS("", "body");

    .parseFragment(aHTMLString, false, null, body));

  return body;

It works by creating a content-level (this is safer than chrome-level) <div> in the current page, then parsing the HTML fragment and attaching that fragment to the <div>. The <div> is returned, and it is never actually appended to the current page. The returned <body> object is of type Element

Here is a sample that counts the number of paragraphs in a string:

var DOMPars = HTMLParser('<p>foo</p><p>bar</p>');

If HTMLParser() returns the element name html (instead of body), you have all document object with its complete functions list, therefore you can retrieve info within div tag like this:

var DOMPars = HTMLParser("<div id='userInfo'>John was a mediocre programmer, but people liked him <strong>anyway</strong>.</div>");

To parse a complete HTML page, load it into an iframe whose type is content (not chrome). See Using a hidden iframe element to parse HTML to a window's DOM below.

Parsing Complete HTML to DOM

Loading an HTML document seems much simpler if it's loaded using the XMLHttpRequest object. For that matter, we're going to load our HTML document first:

var request = XMLHttpRequest();"GET", "", false);

our next step is to create the Document object that will represent the DOM into which we'll insert our newly-retrieved HTML:

var doc = document.implementation.createHTMLDocument("example");
doc.documentElement.innerHTML = request.responseText;

after this any manipulation that we might want to do will be something as simple as the following:

doc.body.textContent = "This is inside the body!";


Using a hidden iframe element to parse HTML to a window's DOM

Sample code may need more work. Create your own function using unique name, ID, and so forth.

var frame = document.getElementById("sample-frame");
if (!frame) {
	// create frame
		frame = document.createElement("iframe"); // iframe (or browser on older Firefox)
		frame.setAttribute("id", "sample-frame");
		frame.setAttribute("name", "sample-frame");
		frame.setAttribute("type", "content");
		frame.setAttribute("collapsed", "true");
		// or 
			// document.documentElement.appendChild(frame);

	// set restrictions as needed
		frame.webNavigation.allowAuth = false;
		frame.webNavigation.allowImages = false;
		frame.webNavigation.allowJavascript = false;
		frame.webNavigation.allowMetaRedirects = true;
		frame.webNavigation.allowPlugins = false;
		frame.webNavigation.allowSubframes = false;

	// listen for load
		frame.addEventListener("load", function (event) {
		  // the document of the HTML in the DOM
			var doc = event.originalTarget;
		  // skip blank page or frame
			if (doc.location.href == "about:blank" || doc.defaultView.frameElement) return;

		  // do something with the DOM of doc

		  // when done remove frame or set location "about:blank"
			  setTimeout(function (){
				  var frame = document.getElementById("sample-frame");
				  // remove frame
				  		// frame.destroy(); // if using browser element instead of iframe
					// or set location "about:blank"
						// frame.contentDocument.location.href = "about:blank";
		}, true);

// load a page
	frame.contentDocument.location.href = ""; 
	// or 
		// frame.webNavigation.loadURI("",Components.interfaces.nsIWebNavigation,null,null,null);

If you are starting with an HTML string, you can convert it to a data URI and use that to load in the browser element.

Using a hidden XUL iframe (alternate example)

Sometimes, a browser element is overkill, or does not meet your needs, or you can't fulfill its requirements. While working on Donkeyfire, I discovered the iframe XUL element, and it is very easy to implement it.

As an example, I will show a browser overlay .xul file, and some JavaScript code to access it.

Here is some XUL code you can add to your browser overlay .xul file. Don't forget to modify the id and name!

<vbox hidden="false" height="0">
  <iframe type="content" src="" name="donkey-browser" hidden="false" id="donkey-browser" height="0"/>

Then, in your extension's "load" event handler:

onLoad: function() {
	donkeybrowser = document.getElementById("donkey-browser");
	if (donkeybrowser) { = "0px";
		donkeybrowser.webNavigation.allowAuth = true;
		donkeybrowser.webNavigation.allowImages = false;
		donkeybrowser.webNavigation.allowJavascript = false;
		donkeybrowser.webNavigation.allowMetaRedirects = true;
		donkeybrowser.webNavigation.allowPlugins = false;
		donkeybrowser.webNavigation.allowSubframes = false;
		donkeybrowser.addEventListener("DOMContentLoaded", function (e) { donkeyfire.donkeybrowser_onPageLoad(e); }, true);

With that code, we obtain a reference to the iframe element we declared in the .xul file. The most interesting piece of code here is the DOMContentLoaded event listener we define for the element. Let's take a look at the donkeyfire.donkeybrowser_onPageLoad() handler:

donkeybrowser_onPageLoad: function(aEvent) {
	var doc = aEvent.originalTarget;
	var url = doc.location.href;
	if (aEvent.originalTarget.nodeName == "#document") { // ok, it's a real page, let's do our magic
		dump("[DF] URL = "+url+"\n");
		var text = doc.evaluate("/html/body/h1",doc,null,XPathResult.STRING_TYPE,null).stringValue;
		dump("[DF] TEXT in /html/body/h1 = "+text+"\n");

As you can see, we obtain full access to the DOM of the page we loaded in background, and we can even evaluate XPath expressions. In the example, we dump() to the console the page's URL and the text contained in the first h1 tag of the page's <body>.

But, we still need to see how to execute the famous loadURI() method using our iframe:

              Components.interfaces.nsIWebNavigation, null, null, null);

Also, I recommend you take a look at the nsIWebNavigation interface.

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 Last updated by: bunnybooboo,