Close TB, then open the profile folder: C:\Users\''username''\AppData\Roaming\Thunderbird\Profiles\''8characterstring''.default AppData is hidden by default. Delete session.json, the file that stores the open tabs at last exit. When you restart TB, right-click the problem message, Delete Message. The message may have locked the program because it was being scanned by a security app. In general, the profile folder should be excluded from AV scanning.