First, install Provider for CalDAV & CardDAV, then launch TbSync, Account Actions, Add new account, CalDAV & Card DAV, select the Yahoo server profile, click Next, and follow the instructions to enter the account address and an app password.

If you have multiple calendars visible, the events from all of them are shown in the same calendar tab.

I tried following this advice because I am having difficulty using my usual method of "discovering" calendars on Yahoo (my credentials are not being accepted for some reason) but when I tried installing latest "provider for Caldav" I am being told that it is not compatible with my version of Thunderbird (102.11.0 64-bit) any suggestions? please

The TbSync/Provider add-ons are no longer needed since TB supports CalDAV. For Yahoo:

"Sync Yahoo Calendar with other CalDAV supported devices. Steps vary by device, but you must enter your Yahoo ID and the app generated password as well as the CalDAV URL associated with your account.

In your device settings, enter https://caldav.calendar.yahoo.com in the server URL field."

https://help.yahoo.com/kb/sync-access-calendar-multiple-devices-applications-sln4707.html

Thank you for replying so quickly. Doesn't seem to make a blind bit of difference

Rather than keep messing up this topic I will continue on my own separate topic (https://support.mozilla.org/en-US/questions/1486702#answer-1704397)