Is the missing folder in your profile name folder? Try the following: Make sure hidden files and folders are visible: * http://kb.mozillazine.org/Show_hidden_files_and_folders#Mac_OS_X In Thunderbird * Help > Troubleshooting Information * click on 'Open folder' button a new window opens showing contents of 'profile name' folder. * Exit Thunderbird now - this is important * click on 'Mail' folder * click on 'Local Folders' mail account. mbox files will have the same name as you see in the Folder pane. mbox files contain emails, they do not have any extension. '''.msf''' files are indexing files - they do not contain emails. Sub folders are located in *.sbd folders. '''.sbd''' folders contain mbox files and index files of subfolders. If you see the mbox file of the missing folder: * Leave the mbox file alone, but make a note of the 'Size'. * delete the *.msf file which shares the same name. This will force a new one to be created. Go back up directory to see contents of 'profile name' folder: delete these files: * 'foldertree.json' * 'panacea.dat' These files will be recreated. Start Thunderbird.