• by Wendy Davis , Staff Writer @wendyndavis, April 24, 2013

Two years ago, Mozilla began offering Firefox users the opportunity to turn on a “do-not-track” signal, which would communicate that they don't want their data collected by ad networks.

As of today, 17% of Firefox users have activated the signal, general counsel Harvey Anderson told a Senate committee today. That comes to 4 trillion do-not-track signals sent by Firefox each month, Anderson said.

Almost all are ignored. That's because the signals themselves don't block tracking. Instead, they communicate users' preferences, but ad networks and publishers are free to treat that information as they see fit. While a few companies -- including Twitter, Jumptap and The Associated Press -- have said they will honor those signals, the vast majority appear to be ignoring them.

Why aren't publishers and ad networks more receptive? After all, over one year ago, the self-regulatory group Digital Advertising Alliance said it would require members to honor the headers, assuming that users expressly turned them on.

At a hearing today, Sen. Jay Rockefeller pressed for answers to that question. Digital Advertising Alliance managing director Lou Mastria, who testified this afternoon, pointed the finger at Microsoft and Mozilla. He said those companies' recent changes to their browsers have hurt efforts to reach a consensus on the subject.

Microsoft said last year that it would turn on do-not-track by default -- a move that the ad industry saw as a betrayal. Industry representatives quickly pointed out that they never promised to honor headers that were turned on by browser developers.

Mozilla said in February that it was going to actually block third-party cookies automatically, at least on a trial basis. Unlike activating a do-not-track header -- which can be ignored -- blocking cookies actually prevents companies from setting them. The ad industry has made no secret of its opposition to that move.

For its part, Mozilla seems to be feeling pressure about the prospective change. Anderson emphasized today that the company is only testing the feature and hasn't made any final decisions about implementing it.

Decisions by Mozilla and Microsoft aren't the only reasons for the standstill. Another is that the Internet standards group World Wide Web Consortium -- a group made up of computer scientists, advocates and industry representatives -- hasn't yet been able to reach an agreement about what “do-not-track” actually means. The main point of contention is whether ad networks should stop collecting data from people who turn on the signals, or only stop serving them targeted ads.

The ad industry says it needs data for analytics and market research, but privacy advocates say that companies should stop gathering data when people say they don't want to be tracked. The W3C is slated to meet again in May.