SUMMARY (AND SOLUTION):

I uninstalled thunderbird and deleted the .thunderbird file (hidden file) in my home folder.

I then reinstalled thunderbird, and when you do so, for each account, you get a choice between two settings, IMAP is the default!:

"IMAP (remove folders)"
"POP3 (keep mail on your computer)"

(What is in parentheses actually appears in the window).

So you have to select POP3 when you add an email account to Thunderbird.

I didn't find a way to alter this setting once an email account has been set up in Thunderbird. This may be possible, perhaps by modifying a configuration file or otherwise, but I couldn't find it, and the confusion created in this thread was due to this difficulty.

The people who responded thought that it was a simple matter of changing the setting in Thunderbird from IMAP to POP3, but in my version or set up of Thunderbird, this simple change was not possible.

My problem is solved, however perhaps someone out there knows how to change this setting after setting up an email account in Thunderbird (configuration file or otherwise).

Many thanks to all.

rm