NoScript

You should enable JavaScript only where you're sure it is safe. Now you can do it with ONE CLICK!

NoScript notifies you about blocked script tags () and Java™+Flash®+Plugin content (J+F+P) using a discreet :-) zap sound and standard "Popup blocker" style messages (you can customize/disable both using the Appearance options).

NoScript provides a really easy whitelist dialog to set JavaScript permissions.

You can customize almost everything in the appearance of NoScript

Advanced options allows you to block Java™, Flash® and other plugins (J+F+P) on untrusted sites (only JavaScript and Java are blocked by default). You may want also to enable rich-text clipboard operation on trusted sites (e.g. your web mail "compose" page).