This is really painless PGP experience! Easy to setup, easy to use

Der email Anbieter GMX empfiehlt diese Erweiterung zur verschlüsselung. Ich vertraue GMX auch nicht zu 100% und nach dieser Empfehlung bin ich stark verunsichert mit GMX was die da empfehlen. Zur Sache Mailvelope will alle meine browserdaten auslesen...kannst vergessen

⚠️Beware of sketchy perms for what should just be an email signer.

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.