Rated 5 out of 5 stars
I've been using Enigmail for years and it keeps getting better and has become much more stable and reliable compared to when I first started using it way back when it first came out. PGP/MIME, per-recipient rules, key exchanges, and everything else works exactly as I needed it to. I am also quite happy that all three of my IMAP accounts managed within Thunderbird are now working well back and forth with my ProtonMail accounts.
I am not sure what trouble some people have getting this to work. Make sure you understand PGP and how its meant to be used, RTFM, and definitely make sure that have your GPGTools (keychain, etc.) working before you try getting this add-on to work for you. A properly configured GPG Tools install and a properly configured Enigmail+Thunderbird accounts results in a in a pretty consistently flawless experience.
Keep up the good work, Patrick!
P.S. An Enigmail email client for iOS (and for my Android friends too) would be pretty rad. Aside from dragging and dropping keys into iTunes file sharing, Canary Mail does not do PGP right.