Hab gefunden, und es funz!
1) Installed T-bird on the Mac.
2) Allowed it to walk me through the setup of one account.
3) Closed Thunderbird.
4) Copied profiles.ini from the Windows install to the Apple install, replacing the one created when T-bird was installed on the Mac.
5) Copied the entire Profiles folder from the Windows install to the Apple install, replacing the one created when T-bird was installed on the Mac.
6) Aside from losing the passwords it seems to work fine.