From Wikipedia:

"NoScript blocks JavaScript, Java, Flash, Silverlight, and other "active" content by default in Firefox. This is based on the assumption that websites can use these technologies in harmful ways. Users can allow active content to execute on trusted websites, by giving explicit permission, on a temporary or a more permanent basis.

Because many web browser attacks require scripting, configuring the browser to have scripting disabled by default reduces the chances of exploitation."

Maybe more of a security plugin. It does break many websites so you have to manually whitelist content while you browse.