Try this: * Right click on Inbox folder and select 'Properties' * click on 'Repair Folder' * click on 'OK' If no joy: Check to see if everything is set to be displayed or whether you have any fiilters being applied. Please check settings say the following: * View > Folders > select 'All Folders' So if you have any other option selected uncheck it for now. * View > Threads > Select 'All ' Just in case some emails maybe set up ignored threads select 'ignored threads' as well. If it proves there are none then you can go back and uncheck the 'ignored threads' option. If you do see ignored threads they will have a red circle icon in the thread column. In which case you need to remove that 'ignored' flag see info : https://support.mozilla.org/en-US/kb/ignore-threads Also check the 'Quick Filter' bar. Are any of the Quick Filter options selected? they will have a thin blue bar appear for anything selected. They used to have a distinctive highlighted background on the button which attracted attention, but now it's just a thin blue bar and you may have missed it. See any blue bars then Click on icon to uncheck and remove filter.