Microsoft announced such a feature for IE9 in December, Mozilla explained similar plans for Firefox and Google introduced a do-not-track extension for Chrome.
As Microsoft, Mozilla proposes an opt-in mechanism for users, which would allow them to choose to opt-out of tracking. The do-not-track information will then be sent by Firefox to a content server that also hosts "tracking" advertising. The ad server then responds with ads that are not personalized and based the tracking information relating to a particular user. Mozilla published a detailed FAQ how it imagines the feature to work and how it could solve common do-not-track problems.
Mozilla said that its idea is merely a proposal and not a complete solution. However, it might be enough for a discussion in the community and among browser developers: "We believe the HTTP header is a constructive approach and one of the many areas we’re exploring to put users in control of their web experience," Mozilla's Alex Fowler wrote.