In your Thunderbird go to menu Help > Troubleshooting Information > Profile Folder > Show in Finder, then quit TB. In the TB profile folder (probably named somewhat like abcd1234.default-release) go to folder "Mail" and open it. You should see a sub-folder "Local Folders". Make a screen shot of the contents of Local Folders.

Thank you Mapenzi for this guidance. I am so grateful.

First I should explain: As of the update, I now have two accounts appearing in my Thunderbird sidebar. One is my email address with Inbox in the dropdown, nothing else. The second is my email address + (IMAP) with all my usual non-local folders. Then below is Local Folders, and it has only Trash and Outbox in it. This is where my many local folders used to be, obviously.

So: I did as you directed. In Profiles, I have a folder called ImapMail. There is no Local Folders folder in it. The other folder, called just Mail, has a folder called Local Folders. My missing folders are not there. Only: msgFilterRules.dat, Trash, Trash.msf, Trash.sbd, and Unsent Messages. Then another folder for Smart Mailboxes.

Does this tell you anything? I can’t imagine why the local folders would just disappear completely. There are hundreds of important emails in them…

Thank you for whatever help you can give.

So: I did as you directed. In Profiles, I have a folder called ImapMail. There is no Local Folders folder in it. The other folder, called just Mail, has a folder called Local Folders. My missing folders are not there. Does this tell you anything?

You searched in your actual profile folder and Local Folders probably don't contain your lost messages, but you should still verify the contents of Trash.sbd .... I have a suspicion that you created an new profile during or after the update. Does the folder "Profiles" in Thunderbird contain more than one or two folders, and if so which are there contents. Specially verify if they contain "Local folders" ! Another possibility could be that you had delocalised your Local Folders in another directory than the profile folder.

Trash.sbd only contains a doc called Unsent Messages. Were you able to access the 2 screenshots? Those show the contents of Mail and ImapMail.

I don’t know how I would have created a 2nd profile, or how I would have delocalised my Local Folders. Are there ways to fix though?

THANK YOU

Trash.sbd only contains a doc called Unsent Messages. Were you able to access the 2 screenshots? Those show the contents of Mail and ImapMail.

Yes, I have seen your screen shots but I couldn't find your missing folders and messages in ..../Mail/Local Folders/....

I don’t know how I would have created a 2nd profile, or how I would have delocalised my Local Folders. Are there ways to fix though?

And since I cannot look over your shoulder on your Mac screen I have to ask for screen shots. Your missing data are not visible in Local Folders, so I continue to search somewhere else and ask questions which you might consider as superfluous or even stupid.

I don't know what happened during your TB update. You describe that "I now have two accounts appearing in my Thunderbird sidebar. One is my email address with Inbox in the dropdown, nothing else." This is not normal! If you only had an IMAP account before the update, where does the second account come from? Did TB after the update start with a request to set up "your existing email address"? But you are very certain that in folder "Profiles" there is no second profile folder that you won't verify?

The contents of the "smart mailboxes" in your "Mail" folder tells me that you are using (or have been using) a so-called "saved search" or other virtual folder with the name "tags". Maybe in your Local Folders you created such a virtual folder containing sub-folders ?

Your questions are great! I appreciate your help and how thoughtful you are about trying to figure it out. I hope nothing I said indicated otherwise.

To answer your questions: 1. If you only had an IMAP account before the update, where does the second account come from? GOOD QUESTION! I have no idea 2. Did TB after the update start with a request to set up "your existing email address"? No. 3. But you are very certain that in folder "Profiles" there is no second profile folder that you won't verify? Yes. 4. The contents of the "smart mailboxes" in your "Mail" folder tells me that you are using (or have been using) a so-called "saved search" or other virtual folder with the name "tags". Maybe in your Local Folders you created such a virtual folder containing sub-folders ? I did not knowingly do so.

I do have Time Machine but not sure where to look. I think this is all beyond me. Perhaps I can find someone to look over my shoulder and help, as you say. So frustrating! But again I thank you for trying to help.

I am grateful to you because since the beginning of this year you are the first person to respond to any of my messages describing solutions and/or asking for more infos... unlike at least 150 other people which didn't bother to comment my messages!

If you have an external hard disk for your "Library" backups please plug it to your Mac and in a Finder window verify if you have made a backup some days before your TB update. I join a screen shot of my TimeMachine backup disk with more or less regular backups of my data, so you know what and where to look for.