I followed instructions to edit the Config File by adding a calendar.registry

What instructions? Can you post a link?

If I click on one of the calendars in the list, then that becomes the default.

I'm not aware of a 'default' calendar concept in Thunderbird. What exactly do you want a default calendar to do or to be?

However, now the default reverts back to Home every time I restart.

I'm still not sure about the meaning of 'default'. Here the calendar selected in the left pane of the Calendar tab is still selected after a Thunderbird restart.

Thanks for responding. The instructions for changing the default calendar are as follows:

"When you have several calendars opened in Lightning, it’s a bit of a pain to double check everytime that the correct one is selected… If you have the same issue, here is the solution :

   Open Tools > Options > Advanced tab, and click Config Editor button.
   In the “Filter:” box enter “calendar.registry”
   Find a .calendar-main-default key – it will be set to true
   Other calendars either won’t have a .calendar-main-default key (or it will be set to false)
   Right click on the value of the .calendar-main-default key that goes with the calendar that currently shows up by default in new events to toggle the value to false
   Click on the .calendar-main-in-composite key that goes with the calendar you want as default
   Right click on the same key and choose Copy Name from the menu that appears.
   Now right click on the key again and select New > Boolean
   Paste the name of the key and Use the backspace key to erase “in-composite” and type “default”
   Click OK and Choose true and click OK
   Now exit out of Thunderbird and restart"

As suggested above, the default calendar is the one that new events are entered in unless you explicitly select a calendar. When the default is Home, then the events are only recorded locally; they do not sync to other devices. I want my Google calendar to be the default so new events automatically show locally but also sync to Google, and through there to my phone and other devices.

As I noted, I am able to make the changes suggested above. They are in the prefs.js file when I open it while Thunderbird is still active. However, when I restart Thunderbird they revert to the old (Home set as default). The prefs.js file is as it was before I made the changes. I discovered that, if you have the list of calendars showing on the left pane and you click the Google calendar, then it becomes the default, e.g. events are entered into the Google calendar without having to select it specifically for the rest of that Thunderbird session. I also found that, when I review the config editor, the changes noted above have been made. However, everything goes back to to Home when Thunderbird is restarted. That is, Home appears at the top of the list of calendars, and it is the default again. Until a month or so ago, one could make the suggested change in the Config Editor and the default would stay on the selected calendar when Thunderbird started. I had thought the issue might be related to TBSync, which I am using to sync to Google, because there was an update about that time that made Google syncing unsupported. However, it now seems that Lightning might have been made to reset the default calendar to Home whenever the code was integrated within Thunderbird (rather than the add-on), which also happened around that time.

I noticed you brought up this problem before. There isn't much more I can tell you other than what has been suggested to you already. https://support.mozilla.org/en-US/questions/1317215#answer-1374448

They are in the prefs.js file when I open it while Thunderbird is still active.

Not sure if this is what you meant, manually editing prefs.js while Thunderbird is running is usually a bad idea.

I discovered that, if you have the list of calendars showing on the left pane and you click the Google calendar, then it becomes the default, e.g. events are entered into the Google calendar without having to select it specifically for the rest of that Thunderbird session.

As mentioned before, the once selected calendar is still the selected one after a Thunderbird restart. So try with a new profile and stop messing with the prefs.js file.

I had thought the issue might be related to TBSync, which I am using to sync to Google

That sounds very unlikely to me.

it now seems that Lightning might have been made to reset the default calendar to Home whenever the code was integrated within Thunderbird (rather than the add-on)

Again, not very likely.

I did not make changes to the prefs.js file while Thunderbird was active. I only viewed the file. I did try changes with Thunderbird not active as a last resort (after I saved the profile). Making a new profile is not realistic as I have more than 5 email accounts with several years of emails and many local folders, also. I cannot afford to start over unless there is some way to retain all that.

I did decide to try moving away from TBSync. The calendars were easy to create using the CalDAV process directly. Now, when I select a calendar as the default, it stays selected when I restart Thunderbird. I also installed Cardbook for my contacts. That also seemed to work well. However, after I created a new contact, it started having problems. Whenever I select a contact from the remote Google address book, the screen now blinks about every second and selecting a contact becomes unstable. It seems to be stuck trying to sync. I placed a note on the Cardbook github page.