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.
[1] https://blog.getadblock.com/the-adblock-family-gets-a-new-ad...