This change – currently available to users running the Nightly test build of Firefox – will bring Firefox in line with its competitor Safari, which has had a very similar policy in place for a decade. It is far from a silver bullet against tracking, as there are several other methods to track users, and this will not block cookies that currently exist in a user's browser. In other words, users must clear their cookies for the new policy to be effective. But instead of just clearing your cookies, for users interested in taking 5 minutes to drastically enhance their privacy, check out our tips for comprehensive tracking protection customizations to your browser.
This move by Mozilla signals that the organization is willing to provide users with much-needed technical countermeasures to tracking, instead of relying solely on the currently stalled development of a W3C Do Not Track standard that appears increasingly unlikely to yield results.
Particularly worrisome are so-called "third-party" cookies. In
rendering an HTML document, a user agent often requests resources
from other servers (such as advertising networks). These third-party
visits the server directly. For example, if a user visits a site
that contains content from a third party and then later visits
another site that contains content from the same third party, the
third party can track the user between the two sites.
Some user agents restrict how third-party cookies behave. For
example, some of these user agents refuse to send the Cookie header
in third-party requests. Others refuse to process the Set-Cookie
header in responses to third-party requests. User agents vary widely
in their third-party cookie policies. This document grants user
agents wide latitude to experiment with third-party cookie policies
that balance the privacy and compatibility needs of their users.
Enhancing user privacy without disrupting user experience may seem like a completely obvious measure to take, but advertisers and other firms have a vested interest in tracking users to serve users with behaviorally targeted advertisements. Since this industry has a lot of influence and money, it is hard to make even the smallest change to the status quo, despite the fact that behaviorally targeted advertising represents only a small fraction of advertising-based business models and countermeasures like these will not hurt ad-supported publishers.