I found a solution: 1. Find the new Thunderbird folder: 1a. From the 'hamburger' menu, select Help > Troubleshooting information. 1b. Under "Application basics" scroll down to "Profiles" and click the link to "about:profiles" 1c Scroll down to "Root Directory" and click the link to "Open Directory." 2. Create a new directory with an extension of ".default" (I called mine "Leon.default") 3. Copy the "profiles.ini" to a different directory (e.g. your own "Documents" directory) so that you can edit it. 4. Use an editor such as LibreOffice Writer to edit "profiles.ini" 5. change the "Path" from "xxxx.default" (where "xxxx" can be a string of numbers and letters) to the name of the new directory that you created (e.g. my new directory was "Leon.default") 6. Save the new "profiles.ini" file. 7. Copy or move the new "profiles.ini" file so that it over-writes the "profiles.ini" file in the new Thunderbird folder. 8. Find your old Thunderbird folder (Mine was in a hidden directory called ".thunderbird"). 9. Find the most recent subdirectory there that has a ".default" extension. It will have a name like "51ymuit0.default." 10. Open that subdirectory 11. Move the contents of that subdirectory into the new subdirectory that you created in step 2 (Mine was called "Leon.default). 12. Start Thunderbird. If everything works, your old emails and settings will be restored.

You shouldn't need to edit ANY files.

It's easy to change the startup and default directory right there in about:profiles.

Do you mean ".thunderbird" was hidden in the OS?

".thunderbird" was a hidden directory, as indicated by the first character being "." The operating system would not allow me to edit the "profiles.ini" file while it was in the ".thunderbird" directory, even if I set its Permissions to Read and Write. I was able to edit a copy of "profiles.ini" after I moved it to a different directory.

When I go to "about:profiles" (as described in steps 1a and 1b above), Thunderbird displays a Root Directory and a Local Directory. Which is the startup directory, and which is the default directory? What should they be changed to? How can they be changed?