Mozilla's plan to prevent third-party cookies by default is on pause. The company this week decided to test blocking third-party cookies by requiring users to enable the feature -- rather than automatically opting them in.

"There are many conflicting claims about how this patch will affect the Internet," wrote Brendan Eich, co-founder of Mozilla.org and CTO and senior VP of engineering at Mozilla, in a post on his personal blog today.

"Why debate in theory what we can measure in practice? We are going to find out more and adjust course as needed. This is the essence of the release test cycle."

The company still intends to release the update with the default non-tracking setting, Mr. Eich wrote, but after further testing. "Our next engineering task is to add privacy-preserving code to measure how the patch affects real website," he said. "We will also ask some of our Aurora and Beta users to opt-in to a study with deeper data collection."

Mozilla caused a firestorm among digital ad industry and privacy stakeholders when it unveiled plans in February to include a default setting that disables third-party cookies in the Firefox browser. At the time, the Interactive Advertising Bureau called the move "harmful to the ad-supported internet across a number of dimensions."

The Mozilla decision was viewed as a victory among privacy advocates, many of whom believe the industry is blocking progress on developing a browser-based Do Not Track standard, a project centralized within the Worldwide Web Consortium. The W3C, as it is known, held in-person meetings of DNT group members this week in California.

"For those who read this as Mozilla softening our stance on protecting privacy and putting users first, in a word: no," wrote Mr. Eich. "The patch as-is needs more work."

Mr. Eich noted Mozilla will provide an update on the initiative "within six seeks."

Microsoft also caused a stir by enabling its Do Not Track setting by default in Internet Explorer 10. Unlike the Firefox patch, which only disables third-party cookies, the Explorer features blocks tracking cookies even from sites that web users have visited.