After announcing that it would soon start blocking cookies from third-party advertisers by default in Firefox, Mozilla has walked back on its plans while it continues to test the system. In a blog post, Mozilla's Brendan Eich said that the patch needed more testing and data in order to refine it for release. "The idea is that if you have not visited a site (including the one to which you are navigating currently) and it wants to put a cookie on your computer, the site is likely not one you have heard of or have any relationship with," he said. "But this is only likely, not always true."

Eich cited false positives — cookies from a site you visit being blocked because it has multiple addresses — and false negatives, in which an ad is clicked accidentally or a formerly trusted site starts adding cookies. The patch, developed by Stanford law student Jonathan Mayer, was initially set to be deployed in Firefox 22. Now, there's no set timeline for its release, but Eich will provide a progress update sometime in the next six weeks.

Advertisers, who depend on third-party cookies to target users across sites, condemned Mozilla for the move, just as they have generally fought Do Not Track efforts by other companies and legislators. Firefox currently has an option to block third-party cookies, but it's previously left off by default. This contrasts with Microsoft, which entered into a protracted fight last year to turn it on automatically in Internet Explorer.