Some of the key security enhancements found in the new version of Firefox are listed below:
Firefox 4 Security Features
Firefox 4 – All Features (Technical writeup)
QUOTE: Like no other release before it, Firefox 4 includes a number of significant security features. These features are addressing attacks that are in particularly hard to avoid by developers and in which the browser is more so the victim then the server.
These attacks, Cross Site Scripting (XSS), redirects to HTTP pages from HTTPS and Clickjacking use vulnerable web applications more as a mirror to bounce attacks into the browser. The browser can provide meaningful protection against these attacks, unlike for more server centric attacks like sql injection, for which the attacker is in full control of the client.