What happens if you open in online mode? Thunderbird has a habit of deleting imap emails that it can not synchronize with the server. It might just be a side effect or being in offline mode.

Hi Matt! When I switch to the Work Online mode, then it does repopulate Thunderbird with all the emails from my active online email accounts. Unfortunately, it doesn't keep my emails for my inactive email accounts. With respect to these old inactive accounts, I downloaded all the online emails in the past one last time before deactivating/deleting the account. I store these for historical purposes in the Thunderbird client. It's strange that everything works fine the very first time I open Thunderbird, but then deletes everything the next time I start up Thunderbird.

Simply .. There is no such thing as an "inactive" account in Thunderbird. It will always try and connect and can always manage to delete what it can not synchronize. But there are some heavy handed ways to make it so.

As you no longer have access to the imap server using the importexporttools to export the emails as MBOX is not an option as it "checks" the status before exporting. https://addons.thunderbird.net/en-US/thunderbird/addon/importexporttools-ng/?src=ss

So what is left is to copy the actual mbox files into the local folders structure in Thunderbird profile so they will appear under "local folders" instead of the original accounts.

So the simplest is to move the entire account files to the local folder in the profile with Thunderbird closed. Locate the imap account server folder.... say "imap.gmail.com" in the imapmail folder of your profile. Copy it to "mail \local folders" folder in your profile and edit the server name part say imap.gmail.com folder name to imap.gmail.com.sdb (sdb tells Thunderbird there is a sub folder)

When you restart Thunderbird your "old" IMAP account will be in local folders with their mails. You can rename the parent folders as you like as the server names will probably not really cut it long term.

Thanks Matt! What I ended up doing was as you said, organizing those "inactive" email accounts with all their emails and folders into local folders. It took a while as Thunderbird only allows moving a limited number of folders/emails at a time. At times it would stop working, so I would need to restart Thunderbird and then was able to move some more batches. Once I had all that transferred to the local mail/folders, I followed the "moving thunderbird data to a new computer" procedures and everything was successful this time.