Did you choose one of the options for Windows 10 preserving your data (e.g., reset, refresh, or possibly similar terms)? You can check for a folder named

C:\Windows.old\Users\your-username\AppData\Roaming\Thunderbird

If you do not immediately see AppData, make sure Windows shows hidden files and folders. To change that setting, see this article (hopefully it's similar in Windows 10): http://windows.microsoft.com/en-us/windows/show-hidden-files

Is there such a folder?

No, I guess I did not. I was unable to find the file . . . AppData\Roaming\Thunderbird in the directory noted.

Have I blown it entirely???

Have I blown it entirely???

Looks like that. Make sure to always have a backup of your Thunderbird profile in the future. http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Backing_Up_and_Restoring

I did find a folder named: c:\Users\Art\AppData\Roaming\Thunderbird that has subfolders

        Profiles\k1vg8m5y.default\

with a number of subfolders: Calendar-data; crashes; extensions; Mail; minidumps; News; and TestPilotExperimentFiles

The "Mail" directory has a number of subs: Local Folders; Local Folders-1; Local Folders-2; Local Folders-3

and LocalFolders-3 has a date modified of 12/7/15 (yesterday)

Is any of this userful, and how do I access it if so???

I did find a folder named: c:\Users\Art\AppData\Roaming\Thunderbird\Profiles\k1vg8m5y.default\

That's most likely your current Thunderbird profile for Win10. You can check this via Troubleshooting Information.

At the top right of the Thunderbird window, click the menu button , then select Help > Troubleshooting Information > Profile Folder > Open Folder.

This will open your current profile in Windows Explorer. I'd expect it to be C:\Users\Art\AppData\Roaming\Thunderbird\Profiles\k1vg8m5y.default\

Nope, it not my current one (per your directions) - the current one is: Users\temp\appdata\roaming\thunderbird\profiles\a3paaj3v.default

Maybe old is recoverable??

That's strange. Your user name changed from "Art" to "temp"? In case this is a transient Windows glitch, have you already tried shutting down and logging back in as Art?

Nope. I had my Windows 7 set so I didn't login each time I rebooted, and it seems that 10 acted the same way - I didn't have to login. (I think). I will try a reboot and see what happens. . .

After a system re-boot (first after installation of Win10), everything was in order and in place. Sorry about my "rush to judgement" and thanks for all the supporting assistance.