Mozilla has added automatic third-party cookie-blocking to a preview version of Firefox 22, a move that will put the feature in most users hands by late June and the company on a collision course with the online ad industry.

Advertising trade groups have blasted the new cookie blocking, calling it "dangerous and highly disturbing," and promising that Firefox users would see more online ads as a result.

On Friday, the privacy advocate who created the blocking code said it had landed on the "Aurora" build channel. "The new Firefox cookie policy has migrated to Aurora!" tweeted Jonathan Mayer, a graduate student in computer science and law at Stanford University.

Mayer is also one of two Stanford researchers who created the HTTP header implementation that signals a user's "No Dot Track" privacy preference.

Aurora is the name for Firefox's least-polished preliminary version that is aimed at general users. In the open-source company's development cycle, Aurora is followed by Beta and then Release. Each edition of Firefox goes through the Aurora-Beta-Release cycles, spending six weeks each in the first two.

Firefox 22, slated to ship as Release on June 25, was moved on Friday to the Aurora channel. Mozilla listed the cookie blocking in its summary of new features for the upcoming browser.

Cookies are used by online advertisers to track users' Web movements, then deliver targeted ads, a practice labeled "online behavioral advertising" by the industry. The new Firefox policy will allow cookies presented from domains that users actually visit -- dubbed a "first-party" site -- but will automatically block those generated by a third-party domain unless the user had previously visited the cookie's site-of-origin.

The by-default setting will not block all cookies and stop all tracking -- in internal discussions, Mozilla acknowledged it was a partial block -- but is designed to slow the explosion of behavioral tracking on the Web.

Even so, discussion at Mozilla to block third-party cookies raised the ire and the rhetoric of ad industry associations last month. Both the Interactive Advertising Bureau (IAB) and Association of National Advertisers (ANA) slammed the new setting, claiming it would force thousands of small online businesses to close, let Mozilla make privacy decisions best left to users, and short-circuit efforts advertisers have made to self-police their ranks.

An ad group representative also said Firefox users would see more ads, not fewer, if Mozilla went through with the plan. "The facts are that [Firefox users] will get more ads, not less, and those ads will not be tailored to their interests. They'll see untargeted ads, which will look like spam," said Dan Jaffe, the ANA's vice president of government relations, in a March interview.

In reply to the ad industry's attacks last month, Mozilla's CTO, Brendan Eich, denied the cookie blocking was imminent. "There will be months of evaluating technical input from our users and the community before the new policy enters our Aurora, Beta and General release versions of Firefox," Eich said two weeks ago in an email. "This will stay in our Nightly build until we are satisfied with the user experience."

Apparently, Mozilla is now satisfied.

Unless Mozilla recants, pulls the feature for technical reasons or simply delays it -- the company has occasionally done the latter -- the blocking will appear on schedule.

Firefox users can disable the cookie blocking, but may have trouble finding the setting. Firefox has tucked it under the "Privacy" section. To negate the blocking, users must select "Use custom settings for history" under the "History" subsection, then change "Accept third-party cookies" from the default "From visited" to "Always."

It's reasonable to assume that few Firefox users will bother.

To try Firefox 22 Aurora, users must download it from Mozilla's website. The browser is available for Windows, OS X and Linux.

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, or subscribe to Gregg's RSS feed . His e-mail address is gkeizer@ix.netcom.com.