Mozilla’s upcoming ‘Do Not Track‘ feature (which allows users to opt-out of the online behavioral advertising) has landed in the nightly build of Firefox. This feature utilizes HTTP Header sent to web servers to tell user’s preference to opt-out of OBA.
To try this, one can install nightly build of Firefox. You can find the option "Tell web sites I do not want to be tracked" in Tools Menu > Options > Advanced > General > Browsing.
This option is disabled by default. Once a user has set this preference, Firefox sends a HTTP header DNT: 1 to web servers with every connection request. This syntax has been recommended by the researchers at donottrack.us also. Though, I’m not sure if the web servers understand this syntax yet.