Storing annotations


Support for extensions using XUL/XPCOM or the Add-on SDK was removed in Firefox 57, released November 2017. As there is no supported version of Firefox enabling these technologies, this page will be removed by December 2020.

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.

Starting from Firefox 53, no new legacy add-ons will be accepted on (AMO) for desktop Firefox and Firefox for Android.

Starting from Firefox 57, only extensions developed using WebExtensions APIs will be supported on Desktop Firefox and Firefox for Android.

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 use WebExtensions APIs if they can. See the "Compatibility Milestones" document for more information.

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


Deprecated in Firefox 29 and removed in Firefox 38.

Warning: this tutorial relies on the since-removed Widget API and no longer works with Firefox.

The widget API is deprecated from Firefox 29 onwards. Please see the ui module for replacements. In particular, for a simple button, try the action button or toggle button APIs, and for a more complex widget try the toolbar or sidebar APIs.

Now we are able to create annotations, let's store them using the simple-storage module. In this chapter we will cover three topics relating to persistent storage:

  • using simple-storage to persist objects
  • handling exhaustion of the storage quota allocated to you
  • respecting Private Browsing

Storing New Annotations

In this section we are only touching the main.js file.

First, import the simple-storage module with a declaration like:

var simpleStorage = require('sdk/simple-storage');

In the module scope, initialize an array which will contain the stored annotations:

if (! = [];

Now we'll add a function to the module scope which deals with a new annotation. The annotation is composed of the text the user entered and the "annotation anchor", which consists of the URL, element ID and element content:

function handleNewAnnotation(annotationText, anchor) {
  var newAnnotation = new Annotation(annotationText, anchor);;

This function calls a constructor for an Annotation object, which we also need to supply:

function Annotation(annotationText, anchor) {
  this.annotationText = annotationText;
  this.url = anchor[0];
  this.ancestorId = anchor[1];
  this.anchorText = anchor[2];

Now we need to link this code to the annotation editor, so that when the user presses the return key in the editor, we create and store the new annotation:

var annotationEditor = panels.Panel({
  width: 220,
  height: 220,
  contentURL: data.url('editor/annotation-editor.html'),
  contentScriptFile: data.url('editor/annotation-editor.js'),
  onMessage: function(annotationText) {
    if (annotationText)
      handleNewAnnotation(annotationText, this.annotationAnchor);
  onShow: function() {

Listing Stored Annotations

To prove that this works, let's implement the part of the add-on that displays all the previously entered annotations. This is implemented as a panel that's shown in response to the widget's right-click message.

The panel has three new files associated with it:

  • a content-script which builds the panel content
  • a simple HTML file used as a template for the panel's content
  • a simple CSS file to provide some basic styling.

These three files can all go in a new subdirectory of data which we will call list.

Annotation List Content Script

Here's the annotation list's content script:

self.on("message", function onMessage(storedAnnotations) {
  var annotationList = $('#annotation-list');
    function(storedAnnotation) {
      var annotationHtml = $('#template .annotation-details').clone();
                                 .attr('href', storedAnnotation.url);
      annotationHtml.find('.url').bind('click', function(event) {

It builds the DOM for the panel from the array of annotations it is given.

The user will be able to click links in the panel, but we want to open them in the main browser window rather than the panel. So the content script binds a click handler to the links which will send the URL to the add-on.

Save this file in data/list as annotation-list.js.

Annotation List HTML and CSS

Here's the HTML for the annotation list:

  <meta http-equiv="Content-type" content="text/html; charset=utf-8" />
  <title>Saved annotations</title>
  <link rel="stylesheet" type="text/css" href="annotation-list.css" />


<div id="annotation-list">

<div id="template">
  <div class="annotation-details">
    <a class="url"></a>
    <div class="selection-text"></div>
    <div class="annotation-text"></div>



Here's the corresponding CSS:

#annotation-list .annotation-details
  padding: 10px;
  margin: 10px;
  border: solid 3px #EEE;
  background-color: white;

#annotation-list .url, .selection-text, .annotation-text
  padding: 5px;
  margin: 5px;

#annotation-list .selection-text,#annotation-list .annotation-text
  border: solid 1px #EEE;

#annotation-list .annotation-text
  font-style: italic;

  background-color: #F5F5F5;
  font: 100% arial, helvetica, sans-serif;

  font-family: georgia,serif;
  font-size: 1.5em;

Save these in data/list as annotation-list.html and annotation-list.css respectively.

Updating main.js

Here's the code to create the panel, which can go in the main function.

var annotationList = panels.Panel({
  width: 420,
  height: 200,
  contentURL: data.url('list/annotation-list.html'),
  contentScriptFile: [data.url('jquery-1.4.2.min.js'),
  contentScriptWhen: 'ready',
  onShow: function() {
  onMessage: function(message) {

Since this panel's content script uses jQuery we will pass that in too: again, make sure the name of it matches the version of jQuery you downloaded.

When the panel is shown we send it the array of stored annotations. When the panel sends us a URL we use the tabs module to open it in a new tab.

Finally we need to connect this to the widget's right-click message:

var widget = widgets.Widget({
  id: 'toggle-switch',
  label: 'Annotator',
  contentURL: data.url('widget/pencil-off.png'),
  contentScriptWhen: 'ready',
  contentScriptFile: data.url('widget/widget.js')

widget.port.on('left-click', function() {
  widget.contentURL = toggleActivation() ?
            data.url('widget/pencil-on.png') :

widget.port.on('right-click', function() {
    console.log('show annotation list');;

This time execute cfx xpi to build the XPI for the add-on, and install it in Firefox. Activate the add-on, add an annotation, and then right-click the widget. You should see something like this:

Restart Firefox, right-click the widget again, and check that the annotation is still there.

Until now we've always run cfx run rather than building an XPI and installing the add-on in Firefox. If the annotation does not reappear when you restart Firefox, double check you installed the add-on and didn't just use cfx run again.

Responding To OverQuota events

Add-ons have a limited quota of storage space. If the add-on exits while it is over quota, any data stored since the last time it was in quota will not be persisted.

So we want to listen to the OverQuota event emitted by simple-storage and respond to it. Add the following to your add-on's main function:

simpleStorage.on("OverQuota", function () {
    title: 'Storage space exceeded',
    text: 'Removing recent annotations'});
  while (simpleStorage.quotaUsage > 1);

Because we use a notification to alert the user, we need to import the notifications module:

var notifications = require("sdk/notifications");

(It should be obvious that this is an incredibly unhelpful way to deal with the problem. A real add-on should give the user a chance to choose which data to keep, and prevent the user from adding any more data until the add-on is back under quota.)

Respecting Private Browsing

Since annotations record the user's browsing history we should avoid recording annotations in private windows.

There's a very simple way to do this: do nothing. By omitting the "private-browsing" key from the annotator's "package.json" file, the annotator opts out of private browsing altogether.

This means that its widget will not appear on any private windows and its selector and matcher content scripts won't run, so the user won't be able to enter any annotations in private windows.

Try it: execute cfx run and open a new private window: you should no longer see the annotator's widget.

Now we can create and store annotations, the last piece is to display them when the user loads the page.