FireGPG is a Firefox extension under MPL which brings an interface to encrypt, decrypt, sign or verify the signature of text in any web page using GnuPG. FireGPG adds some features to the Gmail1 interface, to let you use GPG’s features directly in your webmail. More webmails will probably be supported in the future. FireGPG is able to detect PGP blocks in any page (for example a public key), and lets you easily manage these different blocks.
FireGPG isn’t a key manager. You must install the GnuPG software!
On GNU/Linux and Mac OS, it’s GnuPG. You can install it with your favourite package manager (like Synaptic, YaST, Yum, etc.) or from its official website.
If you are using Microsoft Windows, you have to download WinPT and GPG, and install it at the default location.