You need to copy the messages to Local Folders so that you can then delete the unused account.

1. - click help>troubleshootinginformation
2. - scroll down to 'profile folder' and click 'open folder'
3. - exit thunderbird
4. - assuming POP, click the Mail folder and then the account folder
5. - copy the desired message files, such as inbox, sent, etc - but NOT the msf entries
6. - paste into the Mail\Local Folders folder
7. - restart thunderbird
8. - if all messages are there, you can now remove the old account.

Thank you for the detailed reply David. I was hoping to keep it in the account it is in, as the high level name makes it very clear what it contains (note that it is a VERY old email account and even goes back to a time my wife didn't want her own email account so she used mine on occasion, so she has emails in there she may want to reference). Is there a way I can just stop it from asking for the connection password, given that I (and she) would never be connecting but only perusing?

No. As long as Thunderbird see it as an account, it will attempt a connection. The solution is to move the messages you want to keep out of the account so you can then remove it.

I believe I found away around this. Select the account of interest/concern, click on Account Settings, then Server Settings. There is a section on the right, also called Server Settings. Make sure the following checkboxes are unchecked:

Check for new messages at startup Check for new messages every {#} minutes Automatically download new messages

After making these changes, I shut down Thunderbird and brought it back up. No prompt for the password. I have been in TB for a while and no dialog box has popped up asking for the password. Hopefully this continues over the coming hours, days, etc.

Update: I have not been prompted for a password since I made the above changes. I am considering the problem solved.