Paul wrote:
[...]

As for the availability of the MailboxStore option in the
Server settings, the claim is that you must use this
immediately when the installation of Thunderbird is
brand new.

I think that's not correct. The *installation* doesn't have to be
brand new, the *account* in Thunderbird must be new, i.e. just created.

I added a new New account and could set 'Message Store Type:' to
either 'File per folder (mbox)' or 'File per message (maildir)'.

In my experiments yesterday, I tried to "clean out"
my profile, and tried not to leave any .msf files, then
set the prefs.js with the maildirstore preference, and
that *still* wasn't enough to make it work. I'm going
to have to nuke the damn thing and start from scratch,
to see if I can get it to work.

There's no need to fiddle with preferences as there are perfectly good
settings in the GUI. I think the fiddling might actually have been
counter-productive, because you might have been setting a global default
instead of a account-specific setting.

[...]