I’m much happier to install one of the dozens of adblock addons, than to disable the built in one and still install an addon. Cuz that would mean bloat. I want to have the options to choose the adblock I have installed, and not only which one I use.
Builtin adblockers of other browsers, especially more commercial ones, have proven to be buyable by ad companies. They also fail, and have failed, on YouTube, where some addons still succeed.
One could argue Mozilla could encourage the users to actively choose an adblocker, but that would mean annoying popups and basically ads for adblockers.
Default FF with a few settings and addons is fine.