
The problem has been fixed. Before I removed and re-installed Teams again, I performed a system update. One of the items updated was Firefox, which is now up to v78.0.2. Lo and behold, I can join meetings again.
I did a little digging and discovered that there is now an entry for Teams in the app preferences menu. 78.0.1 didn't have one.
So the fix? Upgrade Firefox.
Thank you all for your help.
Bill