Firefox Web browser to move ahead with "Do Not Track" option Widespread release of the blocking technology remains months away, and no release date has been set. But Mozilla officials spoke confidently Wednesday about the growing sophistication of tools they are building to limit the placement of "cookies" in users" browsers. These bits of code, often placed by data-collection companies that users have never heard of, allow the companies to learn what sites the browser visits over months or even years. Firefox would still allow tracking if users gave a Web site express permission, or if users visited regularly - as is common with shopping, social media or news sites. "We're trying to change the dynamic so that trackers behave better," said Brendan Eich, chief technology officer for Mozilla, a nonprofit group. Its Firefox browser is used by about 20 percent of the world's desktop computers, according to NetMarketShare. The blocking technology that Mozilla is developing borrows heavily from Apple's Safari browser, which blocks all "third-party" cookies, meaning tracking codes from sites that users do not intentionally visit. Mozilla officials say they have refined that approach in recent months to allow third-party cookies in rare cases - for example, when a site that a person visits regularly uses a different Web address, which sometimes is done for security purposes. Mozilla also plans to add limits on cookies placed by sites that users intentionally visit, such as Facebook, to prevent tracking when users sign off and go to other sites. Over the next few months, Mozilla plans to test and further refine its technology before making it available to all users. Randall Rothenberg, president of the Interactive Advertising Bureau, said the changes could disrupt Internet commerce, especially damaging smaller Web publishers that rely on the revenue brought by targeted advertising. He expressed hope that the industry could still persuade Mozilla to abandon or at least significantly alter its plans before widespread release. The Washington Post Co. is a member of the Interactive Advertising Bureau. "There are billions and billions of dollars and tens of thousands of jobs at stake in this supply chain," said Rothenberg, who called the browser makers "oligopolies" with excessive power to make decisions affecting the workings of the Internet. "It should be done with stakeholders" input." Privacy advocates, however, have been forcefully urging Mozilla to push ahead with efforts to block tracking, which they have said could help distinguish Firefox from other browsers in a crowded industry. Mozilla has worked to gain traction in the crucial mobile market, where Apple and Chrome have a natural edge because they build the operating systems for the world's most popular mobile phones and tablets.