Try checking all the following: Check you have this setting: View > Threads > All It's worth also selecting 'Ignore Threads' to see if anything got incorrectly set to ignore. If you then see emails with a red circle in the 'threads' column header you can use info at this link to remove the ignore and then uncheck the View > threads> Ignore threads https://support.mozilla.org/en-US/kb/ignore-threads Is a filter operating? Check the Quick Filter bar to see if anything is enabled. If anything has a coloured thin bar then click on it to disable. Maybe the folder that contains emails needs the index repairing. * Right clickon folder and select 'Properties' * click on 'Repair Folder' button * Click on 'OK' Check the Trash to see if something got deleted. Do you have an Anti-Virus product that is allowed to scan your Profile folders? If yes, then I would advise you access Anti-Virus and set an exclusion to not scan profile folders. You do not want any Anti-virus trying to fix anything as you may lose emails. Do you use a Pop or Imap account ? If imap - Logon to webmail account via a browser to see if emails are on the server.