I think the strongest case for including it in the browser is that the browser is supposed to represent the interests of the user, not the website author.
I'd bet just about anything (though am open to being proved wrong) that when users learn about the pervasiveness of tracking on the internet, they don't like it. Some even take the step of installing add-ons. But since most people don't understand the situation, Mozilla is saying "our goal is to do whats best for the common case user, and even if the user doesn't understand the situation exactly, we don't think they want to be tracked".
And I say, good on them!
