Create a Simple Web Extension

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Much like multimedia support in the browser, we initially did browser add-ons all wrong.  For years each browser had its own language and method for installing add-ons, leading to security exploits, performance problems, painful maintenance for developers, and frustrating user experiences.  Today we have the Web Extension API which is supported by most major browsers, a critical step in alleviating many of the problems developers and users experience today.

At Mozilla's recent all hands event in Hawaii I set out to create my first web extension; I wanted the extension to be useful but simple, something with a real use case.  In the end I created a very simple web extension that continuously monitors the document.title to replace foul words with asterisks, a safety measure to avoid embarrassment when sharing your screen or having people looking over your shoulder.  Let me walk you through how simple creating a basic web extension is!

Extension Structure

The extension must be contained within a single directory -- this is how I recommend the simple extension be structured:

+ foulmouth-filter // (name of your plugin here)
    + icons
        - 48.png
    - filter.js
    - manifest.json

All of the file names and subdirectories can be named how you like with the exception of manifest.json; that is a standard file name which must be at the root of the extension.  The more complex the extension, the more files and structure will likely be needed.


The manifest.json file holds all of the extension properties including images, title, description, requested permissions, which scripts to run on which hostnames, and more.  The following is the very simple manifest I used for my extension:

  "manifest_version": 2,
  "name": "Foulmouth Filter",
  "version": "0.1",
  "icons": {
    "48": "icons/48.png"
  "description": "Filters out filthy words from document titles",
  "content_scripts": [
      "matches": ["<all_urls>"],
      "js": ["filter.js"]

The content_scripts key is important, listing the JavaScript files to include on which hostnames.  For the sake of this simple web extension, I want it to be loaded for every hostname and the meat of the extension will live in filter.js.


The filter.js file contains all of the web extension's logic.  The following code searches the document.title for bad words and replaces them with asterisks:

// Google's bad word filter:
let badWords = "w3schools|david|walsh|jquery.....".split('|') // loool

// Runs cleanup on the document title
let cleanup = word => {
    document.title = document.title.split(' ').map(word => {
        return badWords.indexOf(word.toLowerCase()) != -1 ? '*'.repeat(word.length) : word
    }).join(' ')

// Set up a mutation observer to listen for title changes
// Will fire if framework AJAX stuff switches page title
let createObserver = function() {
    let observer = new MutationObserver((mutations) => {
        // Disconnect the MO so there isn't an infinite title update loop
        // Run title cleanup again
        // Create a new MO to listen for more changes
        console.log('Mutations!', mutations)
        observer = null

        { subtree: true, characterData: true, childList: true }

// Kick off initial page load check

Note:  You'll notice that I tried to use the MutationObserver API to efficiently listen to changes to the document.title but using a MutationObserver made the browser hang so I'll need to look further into how to prevent that -- setInterval is, sadly, the path of least resistance.  I'm confident that my usage of MutationObserver is the problem, not a browser issue.

Installing the Web Extension for Testing

To install and test the web extension in Chrome:

  1. Open Chrome > Preferences
  2. Click the Extensions tab
  3. Click Load Unpacked Extension , navigate to your extension directory, and click Select

To install and test the web extension in Firefox:

  1. Navigate to about:debugging
  2. Select the Add-ons tab
  3. Click Load Temporary Add-on, navigate to your extension directory, and select any file within the extension directory

Web Extension Add-ons Screen

Web Extension Tab

The extension will be loaded in each browser and each new tab's document.title will be checked for bad words.  Each browser has different rules for how long the unpacked extension will be active for, so realize if you end a session you may need to enable the local extension again.  You can learn about publishing your web extension here.

Ideas for Improvement

Here are a few additions that would make this extension more awesome:

  • Ability to add custom words, not just bad words from Google (for users, not the dev creating the extension)
  • Ability to toggle the extension on and off quickly, so that you only filter words when on work hours, for example
  • Publish it to the chrome and Firefox Add-on directories!


The best resource for learning all there is to know about web extensions is MDN.  The web extension we've build here is very simple and doesn't touch any permissions or advanced internal extension APIs, so be sure to read this page to get a complete overview, from start to finish, for creating complete and advanced web extensions.

There you have it -- web extensions aren't nearly as complicated or intimidating as they used to be.  This extension took me roughly 30 minutes to put together so if you have a few spare minutes, give web extension creation a try.  You'll be amazed that cross-browser add-ons have become so easy to create!

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