Firefox 22 will block third-party cookies by default to increase privacy of Firefox users. User privacy is one of the thing that led to the standards like Do Not Track policy, but some sites obey this and others are not, as this features is not mandatory. Browsers also added some features over the years for increasing users privacy, private browsing or incognito mode in Chrome.
Mozilla is testing a patch that will block any third-party cookies but will allow original site cookies. What that means is if you visited a site it’s cookies are not blocked, but cookies set by advertisers using third-party servers are automatically blocked preventing ad networks from tracking user’s behavior to serve behavioral advertising.
Firefox is on a six weeks release cycle, that means major versions of Firefox will be released every six weeks. Firefox 19 is released last week, so you can think of Firefox 22 o release in June this year.