After the install I copied the contents of the old profile into the new profile directory.

Please explain with full path from where you copied to where.

When I go to Help>Troubleshooting Information and select Show Folder

Please provide the full path and name of the profile folder.

The profile was backed up to: / media / Mirror / 8f6qpopn.default /

I copied the contents of that folder and pasted them into: / home / myuserid / .thunderbird / xb3c9d0l.default /

And there was no "Profiles" folder in either case? At some point, all my Thunderbird profile folders shifted down one level into a folder named Profiles.

This is in a Windows system, but you can see the Profiles folder wherein live the abc123.default type folders. I have a similar arrangement under Linux too.

("Address Books" is a remnant of an experiment with an address book export/sync utility.)

Thank you Zenos!

You are correct in your observation. When I installed the new 16.04 system, Thunderbird did NOT create a Profiles directory. I created a new profile in a new directory named Profiles, then copied the old profile information into the new location.

All is well with the world now.