I have the Folder Page View Switcher in, that was working after the upgrade but I innocently changed a setting on it, and it disappeared. I cannot find anything to bring it back.

You no longer need the Folder Pane View Switcher add-on. This functionality is incorporated in modern Thunderbird versions (image). If you can't find your usual display via folder modes start Thunderbird in "troubleshoot mode" and see whether the displays suits you. https://support.mozilla.org/en-US/kb/troubleshoot-mode-thunderbird

Thanks. I removed the folder switcher. I don't use it anyway. I think you'd call the view I normally use "Favourite Folders" where I see my main inbox with favourites underneath, but can still see "All Folders" under that. It all went crazy when TB updated causing a bit of a panic. All good now. Thanks for your response :)