Create button in Thunderbird 60+ plugin

So basically I am trying to create a really simple Thunderbird-Add-On with a button that opens a URL.

So I tried some of the Firefox-Example-Plugins (https://github.com/mdn/webextensions-examples/tree/master/tabs-tabs-tabs) but they all fail because of:

Reading manifest: Error processing browser_action: An unexpected property was found in the WebExtension manifest.

I could not find any documentation how Thunderbird 60+ differs from Firefox here and how to recreate browser_action for Thunderbird

I'm not sure about your specific problem, but let me point to some resources that might help:

• browser_action API is available for Thunderbird 64+.
• Read the Docs page is something closest to official documentation about Thunderbird WebExtension API. Surprisingly MDN will not support Thunderbird.
• Add-ons Update Guide 63+ on Mozilla Wiki is getting updates (although it is focused on existing add-ons).
• Discourse was once the official forum, but didn't gain much traction for add-on developers.
• thunderbird.topicbox.com is the proper and active community as of the end of 2019.

It is expected that you'll find some feature is missing in Thunderbird WebExtension API. Thunderbird developers are extending WebExtension API to match 'legacy' (i.e. current existing) add-on functionality. The plan is to wrap XPCOM feature in WebExtension flavor ("experiments") and call it migrated. They intend to aggregate proposals at GitHub.

Edit 2019-03-02: Thunderbird developer darktrojan added a nice tutorial on add-on conversion and experiments development. It says add-ons can include their own experiment declaration and implementation to use even in stable Thunderbird, so it looks like WebExtensions in Thunderbird can still use raw XPCOM Thunderbird API for now.

Edit 2019-04-24: The documentation is on developer.thunderbird.net.

Edit 2020-01-25: Some quality conversations are on thunderbird.topicbox.com.