Firefox email encryption plugin compatible with PGP

Supports the three major domains Gmail, Hotmail, and YahooMail. The usage method is to first create your own public and private keys, then casually send a test email to a friend to exchange keys. After that, you can communicate via encrypted emails. The main feature of this plugin is that there's no extra operation required when sending encrypted emails. Just click the icon on the Firefox status bar, and as long as you've exchanged keys with your friend, any email you send will be encrypted at the PGP level.