This free Firefox extension, currently in alpha testing, blocks malicious sites at the browser.
Firekeeper is a new security extension from the Mozdev development team for the Firefox browser. The extension is still in alpha, and though it is very stable (it didn't crash our browser), we suggest you bear in mind the code is still in testing. Firekeeper keeps your Internet-browsing session safe by performing as an Intrusion Detection and Prevention (IDP) system at the browser level. It does so by using rules found in the open-source IDP tool Snort.
In practice Firekeeper works like this: When malicious code is found on a site you are about to access, an alert dialog displays the URL of the site, the problem identified, references to security vulnerability reporting sites such as BugTraq, and, finally, actions you may wish to take: Block once, Allow once, Whitelist, or Blacklist. The configuration option allows you to add manually sites to the white or black lists.