HTTPS Everywhere makes Firefox use the SSL-encrypted version of certain websites automatically. It works in the background to redirect your regular HTTP traffic to HTTPS if it’s available. So, for example, when you click a link on http://facebook.com that someone sent you, the extension automatically rewrites your request to connect to the encrypted version of the site instead.
Sadly, the extension only protects you when the websites you access support HTTPS. But it’s a great start for secure web browsing.