Opening HTML mail in a browser is risky due to all the scripting possibilities ... Thunderbird's HTML component allows no scripting and disables loading (remote) images (tracking when I hope the mail) how do you deal with that in such a setup?
(Actually use evolution, but would like to switch to something smaller ...)
I have the "Message Body As" setting set to plain text. This either will display the text/plain part of the message or will render the HTML using plain text markers (* * for bold, / / for italics, _ _ for underscore).
IME, it works out rather well for most HTML emails I receive.
I've banished myself to mobile. I am trying out k9-mail and open keychain on Android. They are both available on f-droid and seem to work well. Open keychain is so much easier to deal with than what I remember of enigmail on the desktop.
OpenKeychain is currently the best way to manage PGP keys on any platform. I don't know why there's no good UI for the desktop, Enigmail is a distant second.