One of the new features that Mozilla introduced in Firefox 4 is a Do Not Track (DNT) setting. When the user enables the DNT option in the browser’s preference dialog, Firefox will transmit a custom header in HTTP requests that will inform servers that the user wants to opt out of Internet tracking. The concept has obvious merit because it provides a simpler, more predictable, and more consistent approach than the cookie-based mechanisms that are currently used today to signify opt-out status.
The downside of Mozilla’s system is that it would be completely ineffective without buy-in from the major advertisers. Despite the fact that it wouldn’t actually do very much, Mozilla offered the setting in Firefox 4 with the hope that it would help build momentum for the superior opt-out mechanism. Its decision to do so seems to have been vindicated. In an announcement, Mozilla revealed that the DNT header is starting to gain high-profile supporters
Read more at Ars Technica