This is an imap mail account, so the account will show what is held in the server folders. To locate which account: * Right click on an imap mail account in Folder Pane and select 'Settings' * Select 'Server Settings' for the imap account. Bottom right, locate 'Local directory:' * Left click in the text area, then repeatedly use right arrow key on keyboard until cursor gets to the end. One of the imap mail accounts will end with this: ......profile\'''ImapMail\imap.mail.yahoo-13.com''' Then you look to the top of same window for 'User name' to discover email address. Why is it so large? Imap mail account folders synchronise with server to offer a view of what is on server. You can choose to only download headers. This means you will not be able to view emails in an offline line state. It alsomeans you do not have a full copy of email on your computer and so cannot perform a backup of emails held on server. But by only downloading headers, you will save a lot on space in the portable thunderbird profile folders. Any email you choose to open is then downloaded to a temp folder to facilitate viewing. * Right click on an imap mail account in Folder Pane and select 'Settings' * select : 'Synchronisation & storage' 'Message Synchronising' section To stop downloading full copies: * uncheck 'Keep messages in all folders for this account on this computer' If you want to keep full messages in specified folders: * click on 'Advanced' * select the folders that you want full downloaded copies. Leave all unchecked if you do not want any full copies downloaded. * click on 'OK' * click on 'OK' Exit and restart Thunderbird. Additional info: * https://support.mozilla.org/en-US/kb/imap-synchronization