The official Thunderbird add-on repository.
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.
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.
The example extension repository from Thundernest with MailExtensions showcasing different WebExtension/MailExtension APIs.
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 of the Mozilla documentation is valid for Thunderbird as well. Useful MDN pages on WebExtension are:
The Mozilla documentation also includes information about internal components and functions used by legacy extensions. Please be aware, that some of those pages are not maintained and are potentially outdated.
An online viewer to search the Firefox code base (mozilla-central) and the Thunderbird code base (comm-central). Useful if you still work with legacy add-ons or WebExtension Experiments and need to check how something is implemented in Thunderbird Core. If you are working on a modern MailExtension, you probably never need this.