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 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 Please be aware, that some of those pages are not maintained and are potentially outdated.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.