I would suggest you import the mbox files (they have no extension) from the old profile into the new profile. This can be done by using an addon tool called 'ImportExportTools'. * https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/ mbox files have no extention. They contain emails. They will have the same name as the folders you would normally see in the Folder Pane. If you have subfolders eg: Inbox has subfolders, then you would see an 'Inbox.sbd' folder, which would contain more mbox files for the subfolders. Do not worry about any file with a .msf extension, these are just indexing files and Thunderbird will create new suitable ones in the new profile. This is the usual location of your profile folder and pop mail account: C:\Users\<Windows user name>\AppData\Roaming\Thunderbird\Profiles\<Profile name>\ Mail\pop mail account name\ '''How to Install Add-on Extension:''' After downloading addon extension *.xpi file to desktop or downloads folder. In Thunderbird ''''Tools' > 'Add-ons' ''' or ''''Menu icon' > 'Add-ons'''' * Click on gear wheel icon and select 'Install Add-on From File'. * Locate the *.xpi file you downloaded and click on 'Open'. * You may need to restart Thunderbird when prompted. To ensure you do not overwrite anything, move all new emails from current new profile folders into 'Local Folders' account. You can always move them back later. then import the mbox files. right click on mail account name in Folder Pane select: 'ImportExporttool' > 'Import mbox files' select: 'Import directly one or more imbox files'. Note: If you need to import an mbox file but it has subfolders you need as well select: 'Import one or more mbox files, with it's subdirectory' click on OK then you need to locate the mbox files and click on 'Open'.