I'd expect this to mean that there are unread messages in subfolders, except that then you should have an asterisk (if the folders are collapsed), which you haven't mentioned.

As I understand it, Thunderbird also makes a non-intuitive distinction between "new" messages and "unread" messages. "New" are, I believe, those that have arrived since you last accessed the account, or maybe the folder, or maybe Thunderbird. I did say it's non-intuitive. But I think blue indicates "new" and black denotes "unread". "New" implies "unread".

Maybe I did not make myself clear. On certain folders, I have a number in parens. The names of these folders are in bold, black text, along with the numbers. There are no unread messages in these folders. If I receive a new message in these folders, the name of the folders and the number of messages are in bold, blue text.

On those folders that have bold, black text with a number in parens alongside, I have read those messages. As an example, Folder A has a number 3 inside the parens. I click on that folder, and there is only 1 message inside that folder. I right-click on that message to mark it as "Read", but that option is grayed out. I close Folder A. The bold, black text goes back to normal. I close Thunderbird. I reopen Thunderbird. Again, Folder A has bold, black text with the number 3 inside of parens. Folder A still has the same message I read several weeks ago. Again still only 1 message inside of Folder A.

right click the folder, select properties and then the repair button.

I right-clicked on the folder, selected properties, and clicked on repair folder on the folders listing unread messages. I closed TB, reopened TB, and the same folders listed in bold with numbers beside the names reappeared. I noticed a selection saying mark folder as read. I made that selection on the miscreant folders. I closed and reopened TB. The same miscreant folders. I went through the miscreant folders, marking the folders as read, then repairing folders to no avail.

Is this a POP or IMAP account?

Is the return of the numbers contingent on restarting Thunderbird?

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It is a POP account. Server Settings: Leave messages on server is unchecked

ScreenShot_1 shows my mailbox with the folders marked in bold with a number beside the folder. If I click on the folder to see any messages, then the typeface returns to normal, and the number disappears. If I close Thunderbird and reopen it, then the same folders with same typeface and numbers reappear. If I just minimize Thunderbird, then open it, whatever state it was in when I minimized it, it stays.

ScreenShot_2 shows a representative folder with the problem, but with no messages inside.

How do messages get into your subfolders under Local Folders?

Do you move the messages by hand? Do you have filters to do it for you?

Or did you move folders there from their original positions in real account folders?