After days my friend found the way to setup Thunderbird to send and receive mail from Riseup, one of the Radical Technology Collectives that work to keep activist communication channels secure. They provide IMAP for downloading your mails, and downloading your mails is the preferred method indeed, even more so if you want to use PGP for encrypting emails as well. Tutorials on their pages were for an old Thunderbird version that didn't look like what we had in the latest Ubuntu. The winning combination:

• Edit > Preferences > Privacy > Security > Verification > Do not use OCSP
• Account Settings > your@riseup.net > Server Settings > Incoming options: IMAP, mail.riseup.net, port 143, TLS, do not use secure authentication
• Account Settings > Outgoing Server > SMTP, mail.riseup.net, port 465, SSL

And the PGP worked (of course with the Enigmail plugin installed) with setting "Use pgp-agent" off:

• OpenPGP > Preferences > Advanced > Do not use gpg-agent.

Ps: At some point i will detail how we configured a PUSCII (another Radical Technology Collective) email address for Emacs and Mew on another friends' computer.