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.