As with standard cookies, third-party cookies are placed so that a site can remember something about you at a later time. Both are typically used to store surfing and personalization preferences and tracking information.
Third-party cookies, however, are often set by advertising networks that a site may subscribe to in the hopes of driving up sales or page hits.
Third-party cookies are often blocked and deleted through browser settings and security settings such as same origin policy; by default, Firefox blocks all third-party cookies. Blocking third-party cookies does not create login issues on websites (which can be an issue after blocking first-party cookies) and may result in seeing fewer ads on the Internet.
Blocking third-party cookies increases user privacy and security but has created a problem for consumer tracking / ad serving firms, which often place ads that follow users around the Web. Mozilla’s decision threatens to make an impact equivalent to the market share of Firefox in their bottom line. Combined with the removal of third-party cookies by other means, some firms estimate that 40 percent of all third-party cookies are removed. Some firms argue that this will affect small business users who rely on third-party consumer tracking and ad serving for revenue.