Also introduced in Thunderbird 38 (38.0.1) was the ability to use Maildir mailbox format. Like OAuth2, this too has not been very well documented. I have no idea what this does or why you would want to use it. Further, I have not tried it, especially after I found this information on the Mozilla Wiki with the warning about enabling said feature:
Though, we aren’t sure of the side-effects of using maildir as the default store on a profile that was created on Berkeley mailbox(mbox) format; so we suggest you to create a fresh profile to work with maildir so as to avoid any dataloss that might occur.
To enable this go into Thunderbird > Tools > Options > Advanced Tab then click the Config Editor… button at the lower right. If you get the ‘This may void your warranty warning”, click the “I’ll be careful, I promise” button to continue. In the search box enter mail.serverDefaultStoreContractID the search will return one result which has a default value of @mozilla.org/msgstore/berkeleystore;1. Double-click the preference and for value enter @mozilla.org/msgstore/maildirstore;1 then click OK. Close the about:config window, click OK on the Options window and restart Thunderbird.