Not hard at all, they could include uBlock add-on by default (as they did with Pocket) except allow people to disable it this time.

They do rely on Google's advertising revenue for multiple $100M of funding; so they presumably don't want to be too successful in democratising ad-blocking.

Eich used to be a controlling influence on FF (as Mozilla CEO), since he was ousted he heads Brave browser which includes content blocking by default (and a content revenue model).

In short perhaps FF/Mozilla are curtailed by financial considerations.