A new study released by researchers at the University of California, Berkeley, and other universites found that the Flash cookies, or local shared objects, are used on 54 of the top 100 Web sites, as ranked by Quantcast. The Flash cookies are stored in a different location than regular http cookies, and are not removed if you delete cookies from within your browser. Per the report, “even the ‘Private Browsing’ mode recently added to most browsers such as Internet Explorer 8 and Firefox 3 still allows Flash cookies to operate fully and track the user.”
While standard cookie-clearing methods don’t get rid of Flash cookies, the report mentions a free Firefox add-on called Better Privacy that can easily nix them. I’ve been using the add-on myself since talking previously with Ashkan Soltani, one of the report’s authors (Shannon Canty, Quentin Mayo, Lauren Thomas and Chris Jay Hoofnagle are the other authors), and currently have it set to automatically delete Flash cookies when I close Firefox.