Je l'ai installé dans mon Firefox mais le serveur de mail que j'utilise me dit toujours que l'extention n'est pas installée ou pas active

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.