May 20, 2023

WebExtensions is a cross-browser extension system designed to allow developers to create add-ons for different web browsers such as Google Chrome, Mozilla Firefox, Opera, and Microsoft Edge, using a single codebase. It is an evolution of the Firefox Add-on SDK and Google Chrome extensions API, which sought to provide a simple and consistent API for writing extensions for web browsers.


WebExtensions is designed to simplify the process of creating and maintaining add-ons for different web browsers. Before WebExtensions, developers had to create separate add-ons for different browsers, each with its own API and requirements. This made it difficult for developers to maintain and update their add-ons across multiple platforms. With WebExtensions, developers can write their add-ons once and have them work across multiple browsers.

WebExtensions also provides a more secure and stable environment for add-ons. Before WebExtensions, add-ons had full access to a user’s browser, which made it easier for malicious add-ons to compromise a user’s system. WebExtensions limit the access that add-ons have to a user’s system, and they run in their own secure sandbox.


WebExtensions are written using HTML, CSS, and JavaScript, which are the standard web development languages. This makes it easy for web developers to create extensions without having to learn a new API or language.

The WebExtensions API provides a set of common APIs for creating add-ons. These APIs include APIs for managing tabs, bookmarks, cookies, and downloads. There are also APIs for adding user interface elements to the browser, such as toolbars, context menus, and notification pop-ups.

Creating a WebExtension

To create a WebExtension, a developer needs to create a manifest file, which describes the add-on and its properties. The manifest file includes information such as the add-on’s name, version, description, and permissions. The manifest file also includes references to the add-on’s JavaScript and CSS files.

Once the manifest file is created, the developer can add JavaScript and CSS files to the add-on. The JavaScript file includes the logic for the add-on, while the CSS file is used to style the add-on’s user interface elements.

The developer can then load the add-on into a web browser for testing. The WebExtensions API provides a set of tools for debugging and testing add-ons in different web browsers.

Publishing a WebExtension

Once an add-on is created and tested, a developer can publish it to a web browser’s add-on store. Each web browser has its own add-on store, such as Chrome Web Store for Google Chrome and Firefox Add-ons for Mozilla Firefox.

When publishing an add-on, the developer needs to follow the guidelines and policies of the add-on store. These guidelines and policies include rules for naming and describing add-ons, and restrictions on the use of certain APIs.

WebExtensions vs. Legacy Extensions

Before WebExtensions, web browsers had their own extension systems, each with its own API and requirements. These legacy extensions were platform-specific and often difficult to maintain across different web browsers.

With the introduction of WebExtensions, many of these legacy extensions are no longer supported. Web browsers are phasing out support for legacy extensions and encouraging developers to migrate their extensions to WebExtensions.