We have about 40 people still using Thunderbird with Exchange Online. One person is seeing many empty grayed-out folders and subfolders in Thunderbird. If you try to delete them, they come back. If you try to unsubscribe the folder, it comes back. They persist from client to client, computer to computer. Logging directly into Exchange Online using OWA does not show the folders. Unchecking "Show only subscribed folders" causes them to disappear." But they still show up if you right click on the account and click Subscribe. I have already opened a case with Microsoft and they say it's a client issue and not an Exchange Online issue.