Used this product happily for years, and contributed occasionally to Mozilla largely because of it. But after the latest sudden, unexpected, forced update, it wet to hell. Used to be I could have subfolders to any folder, like under "Bills" I had each of the organizations that sent me bills and that I was paying. Under "Family" I had those people. Under "Politics" various organization, and various others. Then I could set up filters to place incoming emails into the appropriate folders. Then I had another "pseudo-folder" called "Recent Unread" that showed all unread messages earlier than the 30 days I had selected, and showed what folder they were in. None of that happens anymore. Compared to years of earlier versions, this is junk. I'll soon be looking for another mail client. If this gets any better, I'd appreciate somebody getting back to me (email@example.com) because I might not be here anymore.
Is the problem with Local Folders (POP connection) or with server-based folders (IMAP connection)?
More generally, the Thunderbird support forum is over here: