I use an ad-blocker on Firefox but I think if Firefox started blocking adverts by default it could find itself in a dangerous situation. A lot of sites would feel justified blocking Firefox in that case, which would do more damage to the already-shrinking user base. I know many sites have "turn off your ad-blocker" currently and I usually just leave at that point, but I could imagine a movement to more actively crush Firefox if it had ad-blocking baked in.

Although maybe long term this will happen anyway, so Mozilla has nothing to lose. I mean Chrome will have its ad-tech-friendly blocking-lite, Firefox will be the only real ad-blocking available, and sites will start to make moves to directly discourage Firefox use ("I see you're using Firefox, switch to Chrome to view our crappy site").