Quite a few new posters who have Thunderbird questions actually land on the Firefox Addons board. I’m not sure why; I suspect they have not noticed that TB addons are no longer hosted by AMO, the two sites looking the same.
These are sometimes answered over there, sometimes moved here by somebody like @freaktechnik
For example this Thunderbird WebExtension messages.query -problem which was in the Fx addons board when I responded to it. (I saw it when I was poking about trying to find out how to restore the sticky I quoted! Anybody know?)
So if TB support-seekers - especially those not posting in English - are going to post on Discourse anyway, it would perhaps be best to keep a board here. For example, Lingua info su nuove (e vecchie) estensioni, which is not a developer question. Or maybe it is?
BTW @C-E you selective quoted from the sticky. The first sentence says “This category is for discussing Thunderbird add-ons/extensions.” Not just development.