I changed from POP3 to IMAP and when I reconfigured TB 78.14.0 all seemed good until I tried to call up messages in subfolders of the INBOX. The folders and content are still on the server and it seemed impossible to subscribe to the INBOX in TB. The INBOX itself still functions with new mail and older content showing. The locations of the subfolders had an IMAP mention in the directories that was additional/instead of simply ....'mail.xxxx.xx' but changing to IMAP instead of 'Mail' in the server settings was futile. A deep dive into mozilla resources led to mention of leaving empty the IMAP server directory field (Server Settings>Advanced), and putting "INBOX." (with the following period critical, but quotes not) in the personal namespace field. See: https://kb.mozillazine.org/IMAP:_advanced_account_configuration This worked! However under 'personal namespace' it reverted to double quotation marks ("").