Hi to all,
as I’m new to Thunderbird extension development, can somebody say me what’s the state of the art way to do it? The add-on tutorial is unfortunately marked as outdated. Due to the unavailability of WebExtensions, should I use the Add-On SDK? Or can I only use the Restartless Extensions mechanisms?
I’m thankful for all hints 
Thunderbird has no announced plans to move to WebExtensions, so if the Add-ons SDK works in TB, I would use that. Otherwise, legacy/XUL addons also work. You want to ask someone on the Thunderbird project what their long-term plans are for supporting those technologies so that don’t start a project and have it stop working because of deprecations made by the Thunderbird team.