Version 68 (and 60) are no longer supported. That aside, I suspect this is Bug 1662759 - [Mac] When updating from 68.12.0 to 78.2.1, thunderbird hangs high memory about 2 seconds after start-up while reading from the Mac system address-book See comment 28 of that bug report. Can you confirm?
Bug 1707419 Comment 1 Edit History
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Version 68 (and 60) are no longer supported. That aside, I suspect this is Bug 1662759 - [Mac] When updating from 68.12.0 to 78.2.1, thunderbird hangs high memory about 2 seconds after start-up while reading from the Mac system address-book See bug 1662759 comment 28 for the workaround. Can you confirm?