If I understand correctly, repeat the steps you used to ignore threads. That is, repeating the steps will 'untick' the setting. '

Unfortunately, I don't know what I did to mark the messages, so I can't duplicate the steps.

I hadn't meant to sound flippant. I think you just need to click View>threads>Ignored Threads to untick it.

I thought your reply was helpful, not flippant at all. I appreciate you taking the time to offer help. Using the dropdown menu "View>Threads>Ignored Threads" had no effect. To get rid of the problem, I reinstalled the folder .thunderbird from a backup taken before I inadvertently marked ignore messages. This approach shouldn't be necessary, but trying to apply changes from the Threads' menu didn't work.

Thanks for sharing your solution.