Do Not Track notifies websites and servers you connect to that you do not want to be tracked. What is happening then is entirely up to the companies that run scripts on the website. Those that honor Do Not Track won't track you across web properties while those that do not still will. I do not really like the feature because of this, as you - as the user - do not really know what is happening in the background. It is better to take things in your own hands in my opinion, for instance by clearing third party cookies, HTML5 and Flash storage and the like regularly to make sure tracking is stopped dead in its tracks.
I suppose it is better than not doing anything at all, and it certainly cannot hurt, but unless the system is switched from a honor-bound system to a mandatory one, it is not really helping you a lot as you never know if you are tracked or not.
Mozilla has published a new dashboard that highlights the Do Not Track distribution over time in the world. The countries with the highest Do Not Track count? The Netherlands with 20.41% followed by the United Kingdom with 19.41% and Canada with 18.28% on the desktop, and France with 21.17%, Australia with 19.55% and New Zealand with 19.05% on mobile devices.
What this means? No more than 1/5th of all Firefox users are making use of Do Not Track. It is less than that in most countries though. The desktop average is currently 11.40%, the mobile average 14.11% and it appears as if the values have plateaued in the last couple of months. It needs to be noted that Do Not Track needs to be activated by Firefox users before it is added to the connection header when connections are made in the browser.
How is Mozilla generating the data? It uses the blocklist request that Firefox sends out once per day to retrieve the latest list of blocked extensions, add-ons and plugins. Since this is a request the browser makes, the Do Not Track signal is sent with it, and Mozilla uses it to generate the statistics. It should be fairly accurate as most Firefox users likely have not disabled the feature in the browser (in the about:config dialog by setting the preference extensions.blocklist.enabled to false) or block the request in other ways (e.g. a firewall). (via Sören)Advertisement