Still being always checked in my testing of the 78.2.0 release candidate on Ubuntu 18.04.5 LTS. New profile. Selected an address. Removed the check mark. Closed the address book. Quit and restarted Thunderbird. Checked the address and the check mark was back.
Bug 1655884 Comment 8 Edit History
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~~Still being always checked in my testing of the 78.2.0 release candidate on Ubuntu 18.04.5 LTS. New profile.~~ Helps to test the correct build.