Hi Philippe,
I performed two updates today. On Thunderbird 78.x to version 62 and on Betterbird (91.1.2-bb15) to version 71.9.
Cardbook was opened in a tab. I noticed that this tab got closed. Maybe just by opening the add-ons tab. I am not sure here, but I have seen that closing tab before. Just sometimes, not clearly reproducible. I never took further investigations, because the tab could be reopened again pretty easily.
Now it looks as if immediately after the update one can't get the Cardbook tab back that easy anymore. The menu entry under Tools was missing. It became available again after a restart of Thunderbird.
This is not a big issue, but may confuse users.
Please note, I am using only two add-ons, Cardbook and AHT. So it could be an issue affecting other add-ons as well but I simply did not notice yet.
