Isn't it anti-net-neutrality? Is it ok to build it into a major browser? I actually block all the tracking, malware, ads, social networks, fraud, gambling, porn (except a couple of porn sites I like :-)) etc domains I could find information about but this is my personal conscious choice, I have manually installed extensions for this and built the lists. Shouldn't other people do the same themselves too if they choose…
(Edit: I should add that I think you're asking a very valid philosophical question that my opinion as a user of Firefox doesn't fully address)