re : My default setting is that messages are stored on my system, not my providers.

That would mean you are either using POP or downloading via imap and then immediately moving emails into the default Thunderbird account called 'Local Folders' and storing them in folders under 'Local Folders', so they are no longer in the imap account.

re :Contacted my provider to see if these remained on its server; found a few of the subfolder contents, but the complete folders are nowhere to be found.

But this comment implies you are using IMAP and imap folders and the emails are stored on the server.

So perhaps you mean folders and emails are stored on the server but you were downloading full copies and not just headers, so you could read them even if in offline mode.

Did you ever create a backup of your profile?

What do you have in the profile name folder ?

• Help > Troubleshooting Information

Under 'Application Basics' section

• Locate 'Profile Folder' on left and Click on 'Open folder'

A window opens showing the contents of your profile name folder.

• Click on 'ImapMail' folder
• Click on the imap mail account name folder.

• Make sure you have 'View' > 'Details' selected
• Post an image showing all of the contents.

If you have folders with a .sbd extension:

• click on the *.sbd folder
• Post image showing all the contents.
• Please repeat for all *.sbd folders.

Q: When you renamed the folder, did the rename have a dot or # or ~ in it ? Or were you simply changing eg: lowercase to uppercase ?