I use Pbackup on my PC. It has a TB module that will (also) back-up and restore TB to a different drive ( or even PC If it's installed on that too).

TerryB,

  Thanks but I can't find any Pbackup program on the web. More importantly, I would rather avoid installing third-party software. Isn't there a recommended way within Thunderbird to do what I am after? 
  Thanks in advance.

I read your initial post and shivered. If your editing the raw files you are doing it wrong. Simple as that. Worked in the past, asking for trouble these days.

open the troubleshooting information on the help menu. click on the show profile button. click on the up button in the file manager / Explorer until you see the profiles name (this is generally a sub folder of profiles, in your case probably ytdrvj63.default-release but I am guessing from your broken profiles.ini)

Now copy that folder and all of its contents to the new location. go back to the troubleshooting information and open the about:profiles link. Use the create profile option to create a new profile in the location you just copied your data to and make it the default. Restart Thunderbird.

My experience is that there is yet to be any third party software that reliably manages anything to do with Thunderbird or it's profiles. But this forum is full of horror stories caused by well meaning software deleting important files..

Matt, Thanks for your very clear suggestions, but there is still a problem. I carefully followed every step that you suggested. I successfully (1) copied the ytdrvj63.default-release folder and all its contents to my new E:\Thunderbird\Profiles folder, (2) used the "create profile" button to create a new profile (called g996sfgm.JonE-default) in that same folder, E:\Thunderbird\Profiles, (3) made that new profile FILE (*.JonE-default) the default by clicking the "set as default profile" button underneath the E:\Thunderbird\Profiles folder in the "about:profiles" list, and (4) chose "Restart Thunderbird normally". The only problem is that after TB restarted, it asked to set up my existing e-mail. I did so, but what then appeared was a set-up that omitted all of my dozens of mail folders that contain various types of past sent and received e-mails. Somehow I must have missed a step where the creation of the new profile read that information from the old one. Please suggest a solution to keeping my dozens of mail folders. Thanks in advance.

so when you go to the troubleshooting information and select profile folder, where does it put you? In a different newly created profile folder. So copy everything to the new folder from the old one

Matt,

   Thanks very much. That solved the problem.

I keep my personal/sensitive info on an encryted virtual drive on my HDD created by TrueCrypt where Thunderbird 91.0.1 64-bit is also installed. I wanted to move the TB profiles - which are default placed in the C:\users\>xxx>\appdata\.. etc folder - to this drive. The database files can be read by anybody who knows how to find them. I followed Matt's instructions carefully. It did not work. When I tried to create a new profiles.ini in a new folder on the encrypted drive where the contents of the Profiles folder was already copied, there was no new profiles.ini there, but the original profiles.ini file was modified in the C.\users.... etc. folder. I copied this file to the new folder to the correct place on the encrypted drive. Started TB and it worked, new test-mails were attached to the respective copied files! I renamed the original Thunderbird folder in the C:\users... etc. folder, started TB and then it asked for new username, e-mail etc. which I abandoned. By renaming the Thunderbird folder to it's original name TB worked again. Something is still "hard wired" in the setup. IMHO the developers should create a tool in TB which can do this quite complex copy/move excursion to a simple operation and during a new installation of TB there should be a choice where to place the Profiles folders.

Continued: I moved the Profiles folders from c:\users...AppData.... Roaming and ... Local... to a backup drive, but the Profiles.ini stayed as it was. TB works, and the personal info is to be found only on the encryted virtual drive, which has to be mounted manually when necessary. Awkward "shivering" solution ;-)