Mozilla has announced that, starting with Firefox 22, the popular open source web browser will begin blocking third-party cookies by default. That means only websites you actually visit will be allowed to set cookies; advertisers on those sites will no longer be able to easily track you by setting a cookie.
While there has long been the option to block third-party cookies, by default Firefox has always allowed them.
Apple's Safari pioneered the on-by-default approach to third-party cookies and indeed its third-party cookie policy is still more strict than what Mozilla is proposing. Google's Chrome browser, not surprisingly, allows third-party cookies by default, as does Internet Explorer.
Mozilla developer Jonathan Mayer says the change will "more closely reflect user privacy preferences." Mayer has set up an FAQ for users and developers, but for the most part, given that Safari has always behaved this way, the changes for developers should be minimal.
The main thing to note as a Firefox user is that the change won't affect your current settings, nor will it remove any third-party cookies already set. So to get the benefit of the new policy you'll need to clear out your cookies after you update.
It's also worth noting that, while blocking third-party cookies is a step in the right direction, if you're serious about not being tracked while you browse the web you'll need to take stronger action, installing third-party plugins like Ghostery or DNTMe.
Currently available in the Nightly channel, Firefox 22 is set to arrive in final form in roughly 18 weeks.
