Lewis Rosenthal has modified the Thunderbird extension ImportExportTools version 2.8.0.4 to run under SeaMonkey. This version has support for MailDir storage, and will enable users who opt for MailDir storage in newer versions of SeaMonkey or Thunderbird, and later want to roll back to an older version, to export and then import their messages. The latest version is not yet on the xsidebar site, and can be found via FTP.
Source: OS/2 Warp News and Rumors