To be fair, I would be incredibly surprised if creating a built-in Android content blocker is something Google would even consider, let alone implement.
It is disappointing that they aren't making that API public, but just having that be a core feature of their operating system is pretty huge. Hopefully they have plans to make it public or Firefox will just implement their own ad-blocker into IOS firefox.
Firefox for Android ships with their own browser engine and access to a vast extension library including AdBlock Plus and uBlock. Apple doesn't permit this on iOS which is why Firefox for iOS is using the Mobile Safari engine, doesn't support extensions, and doesn't support ad blocking.
Firefox desktop has numerous adblockers/tracker blockers already available. Mozilla is even sourcing the list from one of the more popular ones, Disconnect. I assume they didn't feel the need to reinvent the wheel in that ecosystem.
Disconnect may be available on Firefox mobile as well, I'd need to check. I never bother beyond uBlock Origin and Self-Destructing Cookies.
It is disappointing that they aren't making that API public, but just having that be a core feature of their operating system is pretty huge. Hopefully they have plans to make it public or Firefox will just implement their own ad-blocker into IOS firefox.