I am voting one-star only to be visible with the bad votes because it almost made me not trying it out:
Contrary to the tone here, it works on Firefox/Linux with native gpg very well (current openSUSE Tumbleweed here).
Follow these instructions: https://github.com/mailvelope/mailvelope/wiki/Mailvelope-GnuPG-integration
Yes, it needs creating one file by hand.

Does not work at all with GPG on Linux.
EDIT: You have to follow https://github.com/mailvelope/mailvelope/wiki/Mailvelope-GnuPG-integration. You may also have to install from GitHub. This was not linked at all on the extension description or in the extension itself, hence the three stars.

FR : Mailvelope est un non-sens. Thunderbird est beaucoup plus pratique et efficace. Fonctionne bien avec Gmail. Mais avec Yahoo et Outlook il n'est pas possible d'insérer une pièce jointe.

US : Mailvelope is a nonsense Thunderbird is much more convenient and efficient.
Work well with Gmail. But with Yahoo and Outlook it's not possible to insert an attachment.

Great tool, super easy to install and use!

version 4.6 works well with Firefox 102.0.1 win64. tested with gmail also.

This is corporate, cosmic nonsense. It's impossible to say anything direct. Good luck with your obfuscations (qv). Just trying to say something......

Now on Ubuntu "Jammy" with firefox 101.0.1. Also the new version 4.6.0 does not work. It can be installed, but is not detected by mail applications. The browser console shows

ExtensionError: No such native application gpgmejson



Just does not work, Ubuntu Impish, FF 100.0.1.

I actually never succeeded in installing this addon with firefox. Sad story.

I'm surprised to see this:
https://outgoing.prod.mozaws.net/
Why use this link in front of other links?
Two examples here:
https://outgoing.prod.mozaws.net/v1/f25582500f72007b6b5fe376d84fdf6185c21ee47d1ddc9a32275ab2f37f7404/https%3A//twitter.com/mailvelope
https://outgoing.prod.mozaws.net/v1/93377538754c52b39b6dbfee793e65c47f34f75a1f1f2b23d27da828005c99c7/https%3A//openpgpjs.org

Installation in Firefox 91.7 on Linux not possible. The installation fails without any error message.

I really wish the GnuPG/Linux would get fixed. This has been an issue for years.
"GnuPG is not available
Do you have GnuPG installed?"