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Extensions are bits of code that modify the functionality of a web browser. They are written using standard web technologies - JavaScript, HTML, and CSS - plus some dedicated JavaScript APIs. Among other things, extensions can add new features to the browser or change the appearance or content of particular websites.

Extensions for Firefox are built using WebExtensions APIs, a cross-browser system for developing extensions. To a large extent, the API is compatible with the extension API supported by Google Chrome and Opera. Extensions written for these browsers will in most cases run in Firefox or Microsoft Edge with just a few changes. The API is also fully compatible with multiprocess Firefox.

In the past, you could develop Firefox extensions using one of three different systems: XUL/XPCOM overlays, bootstrapped extensions, or the Add-on SDK. By the end of November 2017, WebExtensions APIs will be the only way to develop Firefox extensions, and the other systems will be deprecated.

If you have ideas or questions, or need help migrating a legacy add-on to WebExtensions APIs, you can reach us on the dev-addons mailing list or in #extdev on IRC.

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 Last updated by: BryanL,