I'm sad to write a negative review on what I think is an absolutely essential add-on for Thunderbird with no alternative at the moment. Unfortunately it just doesn't work. It's super buggy, there was a major update recently which changed pretty much everything such that the website/documentation is entirely out of date.
As a quick example, it's not possible to edit per-recipient rules; the menu option shown on the website simply doesn't exist anymore. And... those rules don't work anyway. Messages don't decrypt, though they sometimes seem to encrypt, but since it's hot-and-cold I can't really trust it.
I'm not sure if it's the PEP integration that changed everything so drastically but Enigmail simply doesn't work anymore. So sad, I loved this plugin for years. Back to manual gpg on the command line I guess...
Bumping my rating up because of the quick response from the dev and the fact that the plugin is at least basically functional now following his instructions!
I still feel like it's not great that the plugin's PEP integration not only (seems to) break PGP integration, but also reworks the menus so that they're completely different from what's in the documentation... and then the way to get the other menus back is to force a specific mode of encryption (rather than "show advanced menus" or something). Anyway, happy to be leaving a more positive review now.
It looks like you entered the "Enigmail/pEp" mode (which is a simplified mode for beginners). If you want to use the "classical" mode of Enigmail, the go to menu "Enigmail/pEp" > Preferences > Compatibility and select "Force using S/MIME and Enigmail"