WebExtension Documentation for Thunderbird 128

Getting started

These documents assume you already have some familiarity with the WebExtension technology. If not, it is highly recommended to start with the following pages:

For any problems or feature requests please file a bug.


In Thunderbird, all WebExtension API can be accessed through the browser.* namespace, as with Firefox, but also through the messenger.* namespace, which is a better fit for Thunderbird.


WebExtension APIs are asynchronous, that is, they return a Promise object which resolves when ready. See Using Promises for more information about Promises.

The documentation for the APIs listed in the left side panel is generated automatically from Thunderbird’s schema files. The webext-schemas repository can be used to obtain a copy of the relevant files.

Where to get help and more information

Introduction to add-on development

Find information about creating and updating extensions for Thunderbird. Includes getting-started-tutorials and a collection of helpful articles and guides.

Add-on developer community

Learn how to get in touch with other Thunderbird add-on developers, to ask questions and to share knowledge.

Sample extensions

A collection of MailExtensions, showing how to use Thunderbird WebExtension APIs.

MDN sample extensions

A collection of WebExtensions, showing how to use WebExtension APIs in Firefox. They probably won’t work directly in Thunderbird, but they may provide hints on how to use some of the WebExtension APIs that Thunderbird inherited from Firefox.

MDN WebExtension documentation

Find general information about the WebExtensions API cross-browser technology used by Firefox and many Chromium-based browsers. Not all information listed there apply to Thunderbird.