Jonathan Mayer, a researcher at Stanford, has contributed a patch for Firefox that will block third-party cookies from installing on the user's browser. The patch is set to be incorporated into Firefox 22.
With the patch, Firefox would allow all cookies from sites that a user actively visits, but would block cookies from third-party sites if a user has not visited that cookie's origin site. Advertisers generally place third-party cookies and can collect data about a user across several websites with them. This is used to serve more targeted ads or refine where an advertising firm should spend its money.
Blocking third-party cookies would not be new or unheard of among browsers; Apple's Safari already rejects cookies from third-parties. In a blog post on Friday, Mayer called the Firefox patch, "a slightly relaxed version of the Safari policy." Chrome allows all cookies, and Internet Explorer blocks some third-party cookies, although not all. Source: Ars Technica