If the feature is enabled in the user’s browser, the HTTP header will send a “Do Not Track” message to all sites which request their data for behavioural advertising.

The image below shows how the system might work.

The feature is still in proposal mode, however Mozilla hope that all browsers make this an industry standard.

This would be a huge step forward in obtaining user privacy, however if sites do not adopt this standard it will make no difference to web browsing.  If it is accepted by behavioural advertising sites it could give Firefox a great increase in market share!