Review for Enigmail by Sina Momken
Rated 4 out of 5 stars
Having a S/MIME certificate is hard but having a GnuPG keypair is easy. On the other hand using OpenPGP signing without using PGP/MIME messes the email message for those who doesn't have OpenPGP compatible client.
So the best part of this add-on is "OpenPGP Signing + PGP/MIME"
To create your own collections, you must have a Mozilla Add-ons account.