Which folder are you missing? Was it a system folder or one you made? Was it under an account or offline?

The Folder "2024 Monthly Bills" was created by me, under my Thunderbird Acct.

Is it possible that you've dragged it into another folder so that it's become a subfolder? If so, there will be an expansion arrow beside it. Clicking on it will drop the contents.

Sorry no, I physical checked them all as a 1st hope.

Go to C:\Users\[Username]\AppData\Roaming\Thunderbird\Profiles\[default-release]\Mail\Local Folders and then follow the path that's set up in TB to see if you can see the name of the folder amongst the others that you've made.

It'll just confirm if the folder is still there. Somebody else will need to help with how to get TB to recognise it if it is.

Tried your address and it couldn't find the file.

Tried your address and it couldn't find the file.

If the name of the folder is not in or around the place I suggested then, almost certainly, the folder and contents have gone. 'Why?' I have no idea. But, if you can find a file named as the folder was, then I can suggest a possible way to recover it.