alt Hacker NewsAdmiral and several other of the more obnoxious ad networks already claim that Firefox is an ad blocker simply because of its out-of-the-box blocking support for third-party cookies and those ad networks nag you to use another browser. If Firefox added actual ad blocking out of the box I can't imagine the havoc that would cause and how many more websites would claim that they don't work at all in Firefox.
A sharp point in this context because yet again, Mozilla faces contradictory demands in every direction. In today's edition, they are failing their users by not hard coding the ablocking in but also they need to give up on ad blocking because if they try they'll simply be blacklisted.