Earlier quoted context omitted.
It seems to me Android users who want to block ads are a strong target market. Desktop Chrome has extensions and despite the nerf, it has adblockers that mostly work; Android Chrome doesn't have extensions. A built in adblocker would probably help Firefox attract those users, but might destroy their Google revenue stream.
I think the problem with that is that Firefox Android with uBO still feels like it has worse First Contentful Paint than Chrome Android. Even on a high-end phone the difference can feel ridiculous; sites render after 1-2s on Chrome but sometimes I can count up to 5 with FF. The benefits of having uBO might matter more to you and me, but let's not forget that faster rendering was arguably the main reason Chrome Deskto…
I switched from Firefox desktop to Chrome when Chrome was new because it was multi-process and one janky page couldn't hang or crash the whole browser. I vaguely remember the renderer being a little faster, but multi-process was transformative. Firefox took years to catch up with that.
I'm very sensitive to ads though. If a browser doesn't have a decent adblocker, I'm not using it. Perhaps surprisingly, the Chromium browser with good extension support on Android is Edge.