I wonder how up-to-date that warning is, given that the feature was introduced in TB 38 and we're now on TB 60.
Edit: there's still 35 bugs tagged maildir (albeit all unconfirmed or new): https://bugzilla.mozilla.org/buglist.cgi?classification=Clie...
> Maildir is disabled by default because there are still many bugs. It is not 100% ready for users.
Source: https://support.mozilla.org/en-US/kb/maildir-thunderbird