Unfortunately some of the detailed instructions in the Support pages seem to have been written for another version, for example:-
No wizard opens and nowhere is it possible to select Google Calendar.Importing the calendar into Lightning
To import the calendar into Lightning:
Go to the Thunderbird's File menu and select New. From the sub-menu, select Calendar.
Once the wizard opens, select On the network from the available options and click Next.
Choose Google Calendar.
I have tried both the manual instructions for installing Provider for Google Calendar and installing it from an Add-Ons search. The Add-on is showing as installed if I use the latter approach but there is no sign of any wizard or opportunity to select 'On the Network' only from my local directories.
Does this mean that it is no longer possible to use Google Calendar with Lightning?