I understand Mozilla's argument. But I don't agree with it.
DNT has two tracks: Either a small majority of users will enable it and it will be honored by all web sites (highly unlikely) or a large majority of users will enable it and web sites will continue to operate as it currently does. In both scenarios, the end user is relying on a third party to honor a wish.
When it comes down to the almighty dollar, companies have absolutely no incentive to actually comply with DNT. Unless DNT comes with the punishment of fines and can be collected by Joe Sixpack, it will be useless.
If DNT isn't implementable, maybe having a banner that says, "Hey, welcome to my site! By the way, I track what you're doing and sell it to third parties. If you continue on my site, you consent to this." If websites were required to disclose this, then it would be up to the user to either continue on the web site or move on.
Yes, "privacy policies" do this but in legalese and is often obfuscated. I want it so clear that there is no confusion as to what the site records.