Tbird (mozilla-build) just updated to 78. How do I get back to 68?

Since the update about 12 hours ago, I can't use:

Calendar (the "Provider for Google Calendar" addin has apparently loaded OK, buy all my calendars appear empty and their properties pages are all grayed out)

Cardboook ("Incompatible with Thunderbird 78.0")

some tab is just "Loading..." Unclear what it is from position on the tab bar, or why.

etc and my profile has apparently been updated to a new format which is incompatible with the v68 I was using immediately prior.

So I am not feeling very good today. The update just appeared in my ordinary list of mostly security updates and gave no indication it was a major, breaking upgrade.

Please, tell me how I can convert my profile back to 68 format so I can revert to software which met my needs?