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The NoScript Firefox extension provides extra protection for Firefox, Flock, Seamonkey and other mozilla-based browsers: this free, open source add-on allows JavaScript, Java and Flash and other plugins to be executed only by trusted web sites of your choice (e.g. your online bank), and provides the most powerful Anti-XSS protection available in a browser. NoScript is an extension for Mozilla Firefox, designed to prevent JavaScript and Flash scripts from being executed by your browser. This tool can bring extra protection to your browser by blocking clickjacking and other potential threats.

NoScript's unique whitelist based pre-emptive script blocking approach prevents exploitation of security vulnerabilities (known and even not known yet!) with no loss of functionality... It is possible to select the websites you do not want to be affected by restrictions by adding them to a whitelist. It is a very easy to import a list of domains from a text file and you can also export the list if you want to back up the information. All privileges can be revoked if you want to be notified about all blocked items.

You can enable JavaScript, Java and plugin execution for sites you trust with a simple left-click on the NoScript status bar icon (look at the picture), or using the contextual menu, for easier operation in popup statusbar-less windows.
Watch the "Using NoScript" video kindly contributed by John Wilkerson.