Thunderbird does not any auto backups. Have you ever used MozBackup? I would not advise using Mozbackup program as it is out of date and has not been maintained for well over a decade. Even the author advises do so at own risk as it has bugs. Maybe you installed it a while ago and it is still creating backups. Perhaps you have installed 'ImportExportTools NG' ? that can create backups with 'folder name +date' - which sounds like your description. That addon does have a 'Backup Scheduling'. In Menu icon > Addons an Themes > Extensions Locate 'ImportExportTools NG' and click on the spanner/wrench icon select the 'Backup Scheduling' tab Check the settings to see if you set up something.