It 'sounds like' TB lost your profile on the upgrade. I suggest browsing in File explorer to see the actual layout in your profile, which typically is within the appdata/roaming/thunderbird/profile folder. If there is more than one, you can define the preferred one with Profile Manager. To do that, right-click on desktop shortcut key and add -p (after a space) at the end of the Target entry, e.g., "C:\program files\Thunderbird\Thunderbird.exe" -p and the restart TB. From there you use 'create profile' to select one of the profiles you identified.

HI David thanks for response. I have looked for other profiles, but there is only one. The profile is also the only one listed in TB>help>troubleshootinginformation>application basics> profile. Also, it is the one TB opens (which is placed in the C:/ temp folder only while running. ) This profile is not a newly created one, because the create date is the date I started with in 2015 and is an appropriate size. This file size also increases and the date last modified updates every day that I load new emails into TB so it looks like TB is packing the new emails into the same profile mail. I looked for a second mail file inside the profile folder but there is only one. TB can locate my former emails and folders using the TB search which means yes those are in the profile mail file too. For instance, a contact that I have had from 2015 until today is found by TB search, all 900 emails over 7 years. It is just that I cannot then open the searched and found emails, and they still do not show in the normal folder pane. Only the emails I have downloaded since upgrade show. For some reason TB will not load those former emails still contained in the profile folder , but it can find them doing a TB search, but if I try to open one of the searched emails, Tb will not open it. TB will open a window as if it is going to open the email, but then nothing is shows up.

Okay, I've exhausted my TB skills, and maybe someone with more insight on this will see the thread and offer a solution. For me, this is where I drop into technician mode, which may be a rabbit hole where we disappear. Go to File Explorer and locate the inbox, something such as INBOX and INBOX.msf. Use a utility, such as edxor.exe that can open and browse the INBOX file (not the .msf one). Do a search to see if you can locate any info to confirm that a non-accessible email is there. That would confirm that messages you cannot access are in that file. Or maybe there were multiple INBOX files, such as INBOX-1. Our goal is to locate the file where those missing messages are. If they are in the INBOX, you might try the ImportExportTools addon to Extract Folder, having highlighted Inbox first.

Before doing this, it may help to post a screen capture of the File Explorer view of the files in the account folder, e.g., inbox, inbox.msf, sent, sent.msg...

DON'T bother installing the new Thunderbird version (June 2022) because it is not ready for prime time.

Thinking that the process would be seamless, I did download and install it, only to discover that it could not find my profiles, and I would have to proceed manually. To hell with that!

Luckily, I did not uninstall my existing TB software before downloading. I immediately uninstalled the beta version and returned to using my 91.9 version, although I did have to make it the default mail program again.

Windows mail looks like a good alternative in case all goes wrong.