The official Thunderbird add-on repository.
A list of all supported values, which can be used as
strict_max_version values in add-on manifest files.
A guide from the extension workshop with the most recent information on debugging add-ons. It is written for Mozilla Firefox but applies for Thunderbird as well.
A guide how to use Visual Studio Code to debug Thunderbird extension.
Documentation of all Extension APIs created specifically for Thunderbird. They are called MailExtension APIs to distinguish them from the standard WebExtension APIs inherited from Firefox.
Note: Thunderbird Beta supports Manifest V3. The new specification is not finalized yet and will continue to change.
A collection of explanatory sample extensions.
A collection of WebExtensions. 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.
The add-on developer support repository provides useful code snippets, examples and tools in order to better "weather" the transition to the new extension structure of Thunderbird 78.
Overall add-on statistics and a very powerful search in the code of all add-ons listed on addons.thunderbird.net (ATN) and a few other public repositories.
Central bug tracking for Mozilla projects. Some useful pages related to Thunderbird:
The main documentation for Mozilla developers. As Thunderbird is based on the Mozilla platform, some Mozilla documentation is valid for Thunderbird as well. Useful MDN pages on WebExtension are: