I’m trying to figure out how to get my Thunderbird to use Maildir instead of Mbox so my backups are less data intensive.

I opened about:config via Edit -> Preferences -> Advanced -> Config Editor…

Then I changed “mail.serverDefaultStoreContractID” from “@mozilla.org/msgstore/berkeleystore;1” to “@mozilla.org/msgstore/maildirstore;1”.

Everything is easy when you know how!