From MozillaZine Knowledge Base
The “Do Not Track” feature is a mechanism that allows to communicate a user's tracking preferences to websites. User tracking is performed by websites, including advertisers and analytics services as well as social sites, to gather information about the user's web browsing pattern. This includes the history of sites visited along with preferences stated and purchases made. Such sites often use this information to show advertisements for products or services specifically targeted based on the user's profile. See External links below for further reading.
Possible values and their effects
A header stating consent to being tracked is sent to all websites if privacy.donottrackheader.enabled is set True.
A header stating the request not to be tracked is sent to all websites if privacy.donottrackheader.enabled is set True. (Default)
This setting is located in Tools → Options → Privacy:
This setting is located in the Edit → Preferences → Privacy & Security main tab:
First checked in
Has an effect in