While I was doing a manual backup, I closed TB and then accidentally moved (versus copied) the Mail\Local Folders directory out from under my profile.

It is recommended to backup the entire Thunderbird profile. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

However on reopening TB it only shows the default junk, trash, Outbox, and Sent Items folder and their multi-year contents - but refuses to show any of my personal folders under the \Local Folders.

Local Folders is a built-in account, and it get's recreated automatically. Check the 'Local directory' setting for your 'Local Folders' account. At the top right of the Thunderbird window, click the menu button > Options > Account Settings > Local Folders > Local directory

Copy the entire path set for your 'Local Folders' and paste it into your reply. Does it match the folder you copied back after moving it in error?

Christ1, Thanks for the reference, I normally do backup the whole profile - but yesterday was just stupid on my part.

The path was C:\Mail\john\Mail\Local Folders, which did match the folder I copied back.

I did find at least a partial (95%) solution and explanation. The no extension files (as in "example.") files in the root of Local Folders somehow where deleted. The folder I accidental moved to was tied to a cloud account. On recovering from the cloud those files with TB closed, then reopening TB my structure, including subfolders, magical appeared. I believe a feature of the cloud account may have uploaded to the cloud, then deleted the local copy of the files (as a space savings feature).

it is a very bad idea to have your Thunderbird profile in a location that is "synchronized" with a cloud provider. I am assuming the action occurred where ever you placed the backup, but I though I would mention it anyway. The connections to the cloud are simply to slow to not cause problems