They do have a maildir-like format implemented, but I don't think it's yet the d...

		capnrefsmmat on Feb 28, 2015 \| parent \| context \| favorite \| on: Thunderbird Usage Continues to Grow They do have a maildir-like format implemented, but I don't think it's yet the default or easy to convert to: https://bugzilla.mozilla.org/show_bug.cgi?id=58308