Do Not Track is a W3C-driven initiative that lets users inform websites and advertisers that they want to opt out of any advertising that works by tracking their surfing. Visiting Expedia, for example, might show recommendations for London tourist attractions if you've recently visited a site about vacationing in Europe.
The new feature can be found under the privacy and security setting of the Firefox for Android setting panel, and can be activated with a single tap.
Mozilla's Do Not Track implementation works by adding a "DNT: 1" component to HTTP headers. This means every request sent by the user's browser tells the website that the user does not want to be tracked. This approach avoids pitfalls in using privacy cookies, which can be deleted (or simply ignored), and blacklisting, which can be difficult to setup and successfully implement.
Once you've activated the Do Not Track feature, you can use Microsoft's Do Not Track Test Page to confirm it's working.
Users of Apple's mobile devices will have to wait a little longer to receive the same kind of privacy feature. However, Apple recently added a Do Not Track implementation to the desktop version of Safari, to be offered in the forthcoming release of OS X. The signs are good that the feature will soon make it to the mobile version.
DNT does not turn off advertising. Mozilla hopes that websites will simply swap-in a generic ad for any that are targeted at a user's online behavior.
Do Not Track has been spreading since it was introduced with Firefox 4 and Internet Explorer 9 earlier this year. The Associated Press has utilized it on 800 news sites, for example, while the Digital Advertising Alliance (DAA), which represents major advertising agencies, is looking at using it as part of its Self-Regulatory Program for Online Behavioral Advertising.