re :I have created sub-folders within Firefox I presume you mean Thunderbird email client and not mean Firefox as that is a browser. Need info: Is this an imap mail account? Are the emails and folders still on webmail account? If yes, then check if you need to subscribe to see those folders. Right click on imap mail account and select 'Subscribe' to see list. select folders and click on 'Subscribe' click on Ok Is this a pop mail account? If yes, is it set up to use a Global Inbox eg: Local Folders mail account? Are the missing folders in 'Local Folders' mail account? Check you have these settings: via 'Menu bar' toolbar: 'View' > 'Folders' > 'All' or via 'Mail Toolbar' toolbar: 'Menu icon'> 'Folders' > 'All' Check you do not have a filter altering the view eg: Is anything selected in the 'Quick Filter Bar'.