Have you tried highlighting it and clicking 'delete'? You need to share what you have attempted.

Highlighting and clicking Delete does nothing. I have been hesitant about trying anything more radical because my Thunderbird is extremely important to me as an organizational tool.

Another possibility is to exit Thunderbird, use Windows File Explorer and navigate to profile, click to the mail account (Mail folder for POP, Imapmail folder for IMAP), and click the account to list the folders, (e.g., SENT, SENT.msf), to see what folders appear. If the ghost folder appears there, you might try renaming it (to ensure no loss) and then restarting Thunderbird.

I'm embarrassed to say this but I don't know how to navigate to the profile

My error, not yours, for not explaining. To locate it, go to help>moretroubleshootinginformation, scroll down to profiles, click 'about;profiles', and that panel will show you the exact location of your profile. It will be something such as c:\users\<yourid>\appdata\roaming\thunderbird\profiles\<yourprofilename...>. Write that down. Exit Thunderbird, and then locate it with File Explorer. I should have included this earlier.

The link from the incoming e-mail to mark an issue as solved didn't work, so I'm back on the support web page to try here. The important thing is that David's prescription did work, with a little guesswork on my part and a prayer or two. I renamed both the Sent and Sent.msf files, verified that the sent file was no longer in the account folder, and then deleted both. I appear to be good to go. Just tested and my sent messages from that ID are now going to the Sent folder in Local Folders. Thanks!!!

You're very welcome. Thanks for the confirmation.