
To summarise the answer to my own question it sounds like calendar functionality should "just be there" in a fresh Thunderbird installation on Windows, and also if the correct packages are installed on Linux. I suspect that what I should have done (at the very start) on Linux was to install that package and have that available to all users and not install an add-on (for just one user), which caused problems when (a) that add-on stopped being supported any more and (b) I tried to get at a calendar from another user account. Creating New Calendars Using Thunderbird its possible to have multiple calendars. sequence described at may have - it didn't work on Windows when I tried it but I didn't try on Linux). Yes - that did work for the new user account added above on Linux, although it didn't work for the existing user account from which calendar functionality "disappeared" (although the reset/remove etc. Jeli zostanie wywietlony monit, wprowad dane swojego konta Gmail i pozwól dodatkowi na zarzdzanie kalendarzami. Z dostpnych typów kalendarzy wybierz 'Kalendarz Google'. Right-click a blank spot in the Calendar pane. Kliknij menu Plik, wybierz Utwórz i z wywietlonego menu wybierz element Kalendarz.

Apparently, the standard Ubuntu repository didn't include Lightning in TB 60, so you have to install the 'package': Adding a Google Calendar Open up the Lightning calendar tab (click on the Lightning icon in the top right corner of the Thunderbird window).

The support article re update issues has a section for Linux users.
