How lucky that Thunderbird uses the same text mail format for storing messages. All I needed to do was scp individual server directories from under .thunderbird to ~/Library/Thunderbird/Profiles/saltname.default/Mail — that did the job!
I didn’t use the shared global inbox that Thunderbird uses by default. If you do what I did, you probably shouldn’t either.
and better still, it has kept my mail and spam filter settings.