New "feature" since last update 102.10.0 (64-bit): Some, but not all, new/incoming messages are marked with a gold diamond or star icon.

I think it was since version 3 your meant, it is certainly not a new feature.

It is a minor problem, but annoying. How can I stop Thunderbird from doing that?

That star indicates that this is the mail in the list that caused the folder name to change colour as having new unread mail in it which you have not had a chance to see. It is also removed as soon as you move to another folder as you have now seen the new and unread mail.

Note new and unseen is in addition to unread. I have thousands of unread mail in some of my folders, but very few are new since I was last in the folder. I also ass the folder name of folders with "unseen" mail in them in blue and having the same gold star attached to their corner. This also changers when I move to another folder.

Thanks for your answer to the problem, Matt. Basically, a 'feature', so no escaping it.The color change of directory/folder names when one contains unread mail never bothered me, but the new gold star does. Oh, well. I can train myself to ignore the gold stars.

Just FYI, since you help so many people in the Mozilla forum: Thunderbird version in my question was not a typo. This morning, an update brought my copy of Thunderbird up to 102.10.1 (64 bit). I think it likely that the difference is due to this desktop computer running W10. Just out of curiosity, I'll check that by firing up the W11 laptop first, one of these mornings, so I will be able to see the effect of new/unread messages on that machine.

Anyway, I appreciate your taking time to answer my question so thoroughly.