Changes

 This is the same type of addon that Firefox uses. For a general introduction, see [https://developer.mozilla.org/docs/Mozilla/Add-ons/WebExtensions with WebExtensions] and [https://developer.mozilla.org/docs/Mozilla/Add-ons/WebExtensions/What_are_WebExtensions What are WebExtensions]. However, the APIs are different between Thunderbird and Firefox. Some Firefox APIs work in Thunderbird (just try it), other Firefox APIs do not work, and Thunderbird has a few specific APIs added. There is some documentation for the [https://thunderbird-webextensions.readthedocs.io/en/latest/ Thunderbird WebExtension APIs].[https://github.com/thundernest/tb-web-ext-experiments/ WebExtension Experiments] can be used for any API that is not yet provided. There are not many WebExtensions for Thunderbird yet, our own Owl is one of the first that I know of, so you're in new territory. Don't expect a smooth ride. But to be one the pioneers can be exciting, too. Help wanted! More details: WebExtensions have a manifest.json. The following WebExtension APIs are available in Thunderbird: