I have tried to get the development team to even acknowledge this happens. No luck in years of trying.

See https://thunderbirdtweaks.blogspot.com/2019/09/i-lost-my-profilemail-on-update-to.html

Do they want people using a different system instead of Mozilla???? That certainly doesn't make sense.

Don't give up yet. Thunderbird may have just created a new profile, and left the old one alone. If you're comfortable with Windows (assuming you use Windows0, you can invoke File Manager and go to c:\users\<yourid>\appdata\roaming\thunderbird\profiles and see if there are multiple profile folders. If so, look in those profiles in the Imapmail and Mail folders and Mail\Local Folder to see if your accounts are there. If so, we can assist in getting them back.

On your backup question, the safest approach is to use Windows File Manager. To do that: - exit thunderbird (VERY important to avoid a corrupted backup) - copy c:\users\<yourid>\appdata\roaming\thunderbird to external media Then, should you ever need to restore, - with thunderbird installed and not running - copy the backed-up thunderbird folder to c:\users\<yourid>\appdata\roaming to overwrite the existing thunderbird folder.

I do a backup at least weekly and keep the past three copies. And I've never lost any messages.

Yes, the accounts are there. Please advise.

OK, since you were able to locate and view the profiles, I will omit continual references to the locations. I will refer to OLD for old profile and NEW for the active profile. - first, the IMAP accounts will autopopulate, so no work there - The local folders are straightforward.

  - in thunderbird, create a new empty folder for each local folder to be salvaged
 - if not already done, install the importexporttool addon
  - highlight one of the new empty local folders, right-click and select the importexport menu option to import mbox, and then local and select the desired local folder from the OLD profile. 
  - repeat this for each local folder to salvage 

- POP is difficult. The problem is that it may attempt to redownload messages previously downloaded.

   - create the POP account WITHOUT password, as you don't want it to start downloading messages yet 
  - exit thunderbird and manually copy ALL folders from the Mail\popaccountname folder. that will include input, sent, etc and all of the other components, one of which is popstate.dat
  -  restart thunderbird and enter the POP password. If all goes well, the downloads will resume from where you left off. 

Sometimes, things don't go well. But let's hope it does before we take the next step. The moral to us all is to create periodic backups, especially for POP accounts. Good luck.