What error messages do you see. Just remember we only know what you tell us. So statements like " it is not working" do not really add to the collective knowledge needed to determine what the problem is even.

see reply to next comment, please

perhaps have a look in the account settings for those accounts. Check the local directory that is to be used for the account and confirm it actually exists.

again, I am sorry I do not know how best to describe the problem I am having with Thunderbird. If I were some kind of computer IT admin, I might know better. I am uploading an image. There are no error messages. I just can see that for three of the five email accounts, all the folders, (Inbox, Drafts, Sent, Trash, etc) are gone. If an admin updated the macOS and then noticed this problem, what would the admin call this issue? How would the troubleshooting begin? Since I never even set this up to begin with (my admin passed away four years ago), I just am really a beginner. The image should illustrate that two of the five accounts are still "OK" with all their folders, etc. But the upper three do not have arrows, folders, etc. How would the troubleshooting start? In general, on my computer, something happened with the new macOS took over. It moved a lot of my files and folders into a new folder called Relocated items. I am trying to learn what is going on with all that. But I noticed there is a Thunderbird folder in this new folder as well as the one on my desktop.

I will go to the Thunderbird help page see if I can learn what it means "to look at the local directory," and see "whether it exists". I do not know what "confirm whether the account actually exists" really means in terms of Thunderbird. I know the email accounts exist, and that I was using Thunderbird with them for more than ten years until this update. I just want to recover these folders, and the use of Thunderbird for these accounts.

If you right-click on one of the upper three accounts, select Subscribe, click Refresh, does a list of folders and checkboxes appear (see picture)?

I do not see a place to select Subscribe. However, when I select Server Settings, I get an error message now. It says that the local path to the directory is invalid and that I should pick a different directory. I end up needing to force quit after I click OK, because the pinwheel just spins after I select Server Settings.

If Subscribe doesn't appear in the account right-click menu, the account is set up as POP instead of IMAP.

To save you time and energy trying to recover the setup, I would advise you to create a new profile, and add your accounts. After that is working, you can transfer contacts and local mail from the old profile to the new. Help/Troubleshooting, about:profiles, to create and manage profiles. This won't affect the current profile, and either the old or new can be launched from about:profiles.

Hello, and thank you to all who have tried to help me with this problem. To tell the truth, the suggestion to create a new profile and add my accounts, is also way "above my paygrade" LOL. After reading in the Apple support page that some Thunderbird users with the same problem were able to get support from Apple to go back to the older operating system, that was what I opted to do. However, now, I have a new question because now Firefox web browser won't load several webpages, all https related to banking. I can't even log in to Firefox. There is some big problem with cookies and none of the articles are helping me. Why is it so hard to use Firefox and Thunderbird.. I had to use Chrome to access this page.

In the case of Firefox, you may find it helpful to select Help/Troubleshooting, Refresh Firefox, or about:profiles, Create a new profile.

thank you so much! Help - Troubleshooting - Refresh Firefox has solved the issue. I truly appreciate your help!