For the past few months, Firefox alphas have been heuristically blocking certain cookies in a bid to protect user privacy and reduce the amount of online tracking by advertisers. Mozilla has not moved this blocking into the stable builds of its browser, however, because of problems with its effectiveness. The heuristics aren't perfect, so sometimes it blocks cookies it shouldn't block and other times lets cookies through that it should block.
A new project from Stanford University could provide the solution. The Cookie Clearinghouse intends to provide lists of cookies that should be blocked or accepted. Still in the planning stages, it will be designed to work in concert with the heuristics found in Firefox in order to correct the errors that the algorithmic approach makes.
Firefox's algorithm is simple. Essentially, if you visit a domain directly, that domain will be able to set cookies (first-party cookies) and it will continue to be permitted to set cookies even when visited indirectly (third-party cookies). For example, if you visit facebook.com, it will be allowed to set cookies both for explicit visits and whenever other sites embed Facebook content such as like buttons.