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.
A collection of WebExtensions, showing how to use WebExtension APIs. 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.
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 Please be aware, that some of those pages are not maintained and are potentially outdated.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.