What you need to know about making add-ons for Thunderbird.

Current Thunderbird add-ons are based on the WebExtension technology, which is also used by many web browsers. Browser vendors usually refer to their add-ons as WebExtensions. Thunderbird however has a lot of features not available in Browsers, and add-ons written for Thunderbird will most likely not work in Browsers. They are therefore referred to as MailExtensions instead.

MailExtensions are a collection of files that change the way Thunderbird looks and behaves. They can add user interface elements, alter content, or perform background tasks. Thunderbird add-ons are created using standard JavaScript, CSS and HTML.

Key features of MailExtensions:

  • They use stable APIs and do not need to be updated when a new version of Thunderbird is released.

  • They use a permission mechanism that requires users to confirm any permission requested by add-ons before they can be installed. These permission requests enable users to know what an add-on is actually doing.

  • Advanced add-on developers (with in-depth knowledge of Thunderbird source code) can extend the current available set of APIs by creating their own APIs and ship them with the add-ons. These APIs are called Experiments, and they interact directly with Thunderbird's internal APIs, allowing add-ons to use additional features not yet available via built-in WebExtension APIs.

An add-on can either be an extension (adding functionality or changing the way Thunderbird works) or a theme (changing the way Thunderbird looks).

Getting Started

There's a lot of information out there when it comes to add-on development, and finding the most relevant one can be time-consuming. We have therefore put together a tutorial that explains step by step how to create your first extension for Thunderbird:

pageA "Hello World" Extension Tutorial

Add-On Guides

For more detailed information on the two add-on types supported by Thunderbird, see their respective guides:

pageA Guide to ExtensionspageA Guide to Themes

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