To stop the double spaces, go to Preferences > Composition > General and uncheck the option labeled Use Paragraph format instead of Body text by default.

I got the double spacing problem solved, but that still leaves the ????? appearing wherever I've used PopChar to insert a true quote or apostrophe. How do I solve that problem.

You should be using Unicode (UTF-8) text encoding for both outgoing and incoming messages, and the recipient should also be using the same. See https://support.mozilla.org/en-US/questions/1197969

I am using Unicode; always have been.

Which server are you sending on? Some, based on Yahoo, replace special characters with ?? unless a preference in TB is switched.

This is a Yahoo POP account.

See this topic: https://support.mozilla.org/en-US/questions/1260169

AOL, AT&T, Rogers, Sky, Frontier etc. are all based on Yahoo mail.

Does changing the preference eliminate the ??

At first glance, it seems to have solved the problem, but I won't know for certain until I send more email from that account and they reply to me, quoting the original message.

At first glance, it seems to have solved the problem, but I won't know for certain until I send more email from that account and they reply to me, quoting the original message.

You can use another email account for such tests, for example a Gmail account, then send to it and reply from it.