Read and edit Thunderbird's maildir with GNUS

As forums tell it should be possible to use GNUS for reading maildir created by Thunderbird. I did not find, however, any working implementations posted. And in my own experiments I could not make GNUS to list groups (folders in maildir) to subscribe to.

This is what I did:

• Manually set up gmail account in Thunderbird and selected the option Message Store Type as File per message (maildir)

• Checked Thunderbird's directory where it should store emails in maildir format. In my case it is /home/username/.thunderbird/some-id.default/ImapMail/imap.gmail.com which contains the following:

  drwxr-xr-x 4 4k 01-26 10:36 Archive drwx------ 6 4k 01-26 10:49 [Gmail].sbd drwxr-xr-x 4 4k 01-26 10:36 INBOX -rw-rw-r-- 1 196.7k 01-28 12:28 Archive.msf -rw-rw-r-- 1 1.1k 01-26 10:30 Drafts.msf -rw-rw-r-- 1 2.5k 01-26 11:00 [Gmail].msf -rw-rw-r-- 1 973.4k 01-28 12:22 INBOX.msf -rw-r--r-- 1 25 01-26 10:30 msgFilterRules.dat -rw-rw-r-- 1 2.8k 01-27 12:28 Outbox.msf -rw-rw-r-- 1 1.1k 01-26 10:30 Sent.msf -rw-rw-r-- 1 1.2k 01-26 10:36 Templates.msf -rw-rw-r-- 1 2.8k 01-26 10:59 Trash.msf 

• Then I set the following GNUS options:

  (setq gnus-select-method '(nnmaildir "" (directory "/home/username/.thunderbird/some-id.default/ImapMail/imap.gmail.com") (get-new-mail nil))) (setq mail-sources nil) (setq gnus-secondary-select-methods nil) 

• M-x gnus and press ^ (gnus-group-enter-server-mode)
• Select {nnmaildir:} (opened) and press RET
• ... and see nothing (there are no groups)
• ... also nothing to subscribe to in *Groups* buffer when I press U (gnus-group-unsubscribe-group)
• Tried to change permissions for imap.gmail.com/ (Thunderbird's maildir directory) with chmod -R +w+r+x imap.gmail.com/. Same results.

What is wrong? Why does not GNUS see the groups (mail directories)? How should I set up GNUS to read and edit Thunderbird mails?

(I am on ubuntu 16.04 LTS with Emacs 25.1.1 and Gnus v5.13)