Thunderbird is telling me that there is an unread message and it is one I read several days ago, listed in bold type again.
Every day or so Thunderbird says there is an unread message. I look for it and find it on the list of my messages received. It is in bold type as if it is unread. However I had read the message days ago. I had this problem a couple weeks ago and decided to delete all the messages in my inbox. That solved the problem for a couple weeks, now it is back. I thought at first it was because my inbox was getting overloaded but now there are only a couple hundred messages in it. Maybe I should just uninstall Thunderbird and then install it again.
Wšykne wótegrona (1)
remove all mails from the inbox (Copy them to another folder or delete them. It makes litle difference.
Now Select File menu (alt+F) > compact folders.
I am guessing there is some corruption hiding away in there. A compact should fix it, but will do it better on an empty folder.