Much to the dismay of the advertising industry, Firefox announced this week that the next iteration of its Mozilla browser will block third-party cookies by default, effectively cutting off their ability to track users.

Mozilla, which accounts for roughly 20 percent of the desktop market, said the change will be made with the release of Firefox 22 in April. While the browser will allow cookies from first-party sites that are visited by users, third-party cookies will be blocked by default unless there is an existing relationship with the user.

Mozilla’s privacy officer Alex Fowler wrote in a blog post that the change is the result of several factors. “Many years of observing Safari’s approach to third party cookies, a rapidly expanding number of third-party companies using cookies to track users, and strong user support for more control is driving our decision to move forward with this patch,” he wrote. The company will test the function for several months, he added.

Privacy advocates commended the move, with Center for Digital Democracy executive director Jeff Chester praising Mozilla for demonstrating a “serious commitment” to online privacy.

The ad industry reacted with horror.

In a tweet, Interactive Advertising Bureau general counsel Mike Zaneis called the new policy “a nuclear first strike against the ad industry.” Stu Ingis, counsel to the Digital Advertising Alliance, told MediaPost the patch would “negatively impact the entire Internet ecosystem” and was “a horrible thing for consumers.” “Is everybody just going to work for free?” he asked.

Why it matters: Mozilla’s decision follows last year’s “nuclear strike” when Microsoft introduced Do Not Track as a default setting for Internet Explorer, creating a firestorm in the ad industry and resulting in a letter of protest signed by the Association of National Advertisers and the chief marketing officers of more than 30 companies, including General Electric, Ford, and Kraft. While the Federal Trade Commission and privacy advocates praised Microsoft’s move, the Do Not Track movement stalled last year. Members of the World Wide Web Consortium’s working group failed to reach an agreement on even basic issues, like how to define DNT. But movement now seems to be swinging in the opposite direction as law professor Peter Swire, co-chair of the working group, recently stated in a blog post entitled “Full Steam On Do Not Track.” After a recent meeting, the group now has “a roadmap” for a DNT standard, he said. “We are now on a path to devising a workable, meaningful standard.”