I manage four email accounts with Thunderbird. The unread messages count in the InBox for two of the accounts is correct, but the other two show one unread message each, but there no unread messages in the the displayed list. Matt recommended InBox==>Properties==>Repair Folder to someone else with the same problem, which I tried but which did not resolve the issue in either account.

This is not a major issue and doesn't really have much effect on usability, but it is annoying and I'd love to have it resolved.