NoScript Security Suite for Firefox

Description

It allows JavaScript, Java and other executable content to run only from trusted domains of your choice, e.g. your home-banking site, guarding your "trust boundaries" against cross-site scripting attacks (XSS), cross-zone DNS rebinding / CSRF attacks (router hacking), and Clickjacking attempts, thanks to its unique ClearClick technology. It also implements the DoNotTrack tracking opt-out proposal by default.

Such a preemptive approach prevents exploitation of security vulnerabilities (known and even unknown!) with no loss of functionality where you need it.