FireGPG is an awesome little plugin that adds GPG support to Firefox. You need the GPG package installed on your machine to start, and after activating the plugin, you’ll have a new right-click menu that will let you sign, encrypt, decrypt and verify any selected text.
You can use this to add strong crypto functionaliy to any webmail system or forum that you use! Special support for GMail is already built-in, which provides encryption and signature buttons right alongside the normal send button.
Currently, there isn’t a lot of documentation, but the author has set up a Wiki. If you want to help out, try the software for a while and pitch in with a page or two on the maual.
GPG (GNU Privacy Guard) for Linux, Mac, and Windows – Link