If you can't wait and want to use a Mozilla build of Firefox, see the guide for installing Mozilla builds.
Dovecot is a Mail Delivery Agent, written with security primarily in mind.
It supports the major mailbox formats: mbox or Maildir. This guide explains how to set it up as an IMAP or POP3 server.
Dovecot will NOT work in an encrypted directory/folder.
We will be releasing Lightning 2.6.1 this week to fix the issue and afterwards find out how this could have happened.