Strange behaviour when copying profile

I just upgraded to MacOS "Catalina" and wanted to make a backup of my default "user Profile" for Thunderbird 60.9.0. As I have always done in the past, I went into Profile Manager, created a new profile, and then went into the Library, copied all the files from the old profile folder into the new one. But now, when I launch Thunderbird under the new profile, I find it has created a bunch of new folders such as "inbox-1", "sent-1", etc. instead of just "inbox", "sent"... And some of my files are in "inbox-1", "sent-1" while others stayed in "inbox", etc. After opening and closing Thunderbird a few times, resetting and restarting, and trying it twice with different methods of "copying all the files" to the new profile folder (to make sure I wasn't just accidentally skipping a file during the transfer, or something), I just opened Thunderbird again under the new profile, and all these folders have disappeared completely... (even my "archive" folders, and my inbox's folder structure...) it's weird... Launching into the 'old' profile somehow still gives me the correct set of folders that I always had (thank God!)... but this new profile behaves strangely. I think this is a bug, and I think it might have something to do with my having 'upgraded' to Catalina. Thanks for your ideas...