View > Sort by > Unthreaded Have a look at this older question because I offered a load of information on setting up defaults. https://support.mozilla.org/en-US/questions/1422833 Basically developers have forced some defaults which have not been well received by some people. You may need to reset the preferences for wanting new folders to use 'unthreaded' and 'descending' as default. There is another easy way to see all unread in a selected folder. This works very well particulary if using threaded view because it immediately removes all the clutter. It's also handy of you've missed an email which may be scrolled out of view. Use the Quick Filter toolbar Click on the 'show only unread mesasges' envelope icon click on it again to go back to normal view.