When Mailvelope works it's beautiful and easy to use. I confirmed it pretty easily by sending PGP encrypted messages between my GMail and Protonmail accounts.

What's not great is that it doesn't work in Firefox containers. This is a pretty big downside because it means that to get privacy for your email you need to log into Google in your "No Container", which allows Google to follow you around the web (unless you do everything else in containers, which is annoying). If this extension supports Firefox containers in the future, I'll be sure to give it another look.