This is a repost from EFF (Electronic Frontier Foundation)
Technical Analysis by Peter Eckersley
This Firefox extension was inspired by the launch of Google's encrypted search option. We wanted a way to ensure that every search our browsers sent was encrypted. At the same time, we were also able to encrypt most or all of the browser's communications with some other sites:
Firefox users can install HTTPS Everywhere by following this link.
As always, even if you're at an HTTPS page, remember that unless Firefox displays a colored address bar and an unbroken lock icon in the bottom-right corner, the page is not completely encrypted and you may still be vulnerable to various forms of eavesdropping or hacking (in many cases, HTTPS Everywhere can't prevent this because sites incorporate insecure third-party content).