Noscript

NoScript is a popular Mozilla Firefox extension that enhances Web browser security. By default, it prevents the browser from executing JavaScript and plug-in content from any domain unless that domain is expressly allowed. Although it may (initially) prevent AJAX-driven Web sites from working until the main domain and its content delivery domain are permitted (such as twitter.com and twimg.com for the Twitter service, and youtube.com and ytimg.com for the YouTube service), it also prevents scripts from untrusted third parties (such as ad networks) from running even after the trusted domains are permitted. Scripts can be allowed temporarily or permanently on a domain by domain basis, and domains mistakenly allowed can have their permissions revoked, from simple clicks of NoScript's option menu.