Web plugins such as Flash, Java and JavaScript can be used to add a range of interesting features to web sites, but there are also a number of downsides. In addition to reducing the performance of sites, there is also the potential for scripts to be used for malicious purposes. NoScript is a Firefox addon that can be used to prevent the running of scripts on the sites you visit.

The addon automatically blocks all such content, but there may well be occasions when you need it to run. When you encounter such as site, you can easily add it to a whitelist so that scripts will be allowed to run now and in the future - ideal for banking and shopping sites.

You can choose to be notified when a script is blocked, so should a site fail to function as expected you can opt to enable individual page elements as required. The settings you choose to use can be quickly backed up by saving them as a bookmark so they can be restored when you have to reinstall, or transferred to another computer.

With hardly any user intervention required, NoScript offers a superb level of protection against site borne security threats and ensures that you have full control over what is allowed to run in your browser window.

Version 2.9.0.12 (Changelog):

x Updated DNT implementation to match the most recent spec
  about navigator.doNotTrack values (thanks Francois Merier)
x [XSS] Better compatibility with Unionbank's website (thanks
  Brent for reporting)
x Fixed bug 1278735 (JavaScript disabled in private windows)
x Fixed JSON viewer not working
x about:feed in the mandatory whitelist to fix bug 1272139
x [XSS] Disable JavaScript on FTP-served pages when a
  potential  DOM XSS threat is detected (thanks Emanuel
  Bronshtein @e3amn2l for reporting)
x Fixed DOS through script-triggered ClickToPlay confirmation
  dialogs in a loop (thanks Emanuel Bronshtein @e3amn2l for
  reporting)
x Fixed placeholder links might be potentially used as XSS
  vectors if stars were properly aligned(thanks Emanuel
  Bronshtein @e3amn2l for reporting)
x [Surrogate] Updated google-analytics.com replacement (
  thanks noscriptsplox)
x [XSS] Fixed regression (thanks Masato Kinugawa for report)

Verdict

A small and simple addon that places you firmly in control of the web sites you visit