What do they need to deny? Ad blocking is clearly a feature many users want. It’s often the only extension people install. If I were Firefox, I’d build it right into the core of the engine. Turn it on with a checkbox and/or custom blocklist sources.

Yes, I meant building it in by default won't remove any deniability, because it will just be a content blocker where users can load their own filters (with maybe some adblocking filters suggested by default).