Because ad blockers are just add-ons, they can be circumvented. Also, many ad blockers allow advertisers to pay them to allow their ads to get through: https://webcache.googleusercontent.com/search?q=cache:G3lu90...
Although Mozilla has had it's own questionable stuff going on, I still believe in the Mozilla organization. We need browser engine diversity, and with Microsoft Titan/Edge engine going away, the only alternative to Blink/Webkit variants is Gecko (I wish Microsoft would just open source their Edge engine).
I honestly don't want all that shit in the core of the browser. They should be extensions, so you can take the things you want. It's why Jenkins seems like crap when you first use it compared to other build tools, until you leverage the ecosystem of Jenkins plugins.
Your second sentence is true for adblock plus, but not ublock origin
I actually drop people on Patreon when they start adding ads or sponsors into their podcasts or videos. I would rather people be fully supported by their fans instead of double dipping into both fan support and adverts.