Thanks davidb for your response. Thunderbird no longer supports this since 78.3 (I think). I have been waiting and hoping for the add-on to be updated, but nothing yet. Perhaps I should look for the configuration to see if I can edit it directly? I mean, If I find it, it should be readily recognisable, shouldn't it? Other TB add-ons work, such as Google calendar and mail-merge, which I love.
So you can see why I am still unable to make use of my Postbox licence.