If you have a phone without GApps, you're either in China (which has its own app store ecosystems) or not Mozilla's target audience for their binaries.
Again, resources. My workstation is still waiting to get torn down to have the smoke scrubbed out; my desk is torn down and cleaned already but still waiting for me to have a new place to put it. My room didn't burn but there's no electricity and the whole place is not habitable; by the time it is, it won't be my room anyway.
The 4GB laptop isn't very good for compiling large projects, because running WF needs much of that-- and then, just linking libxul.so needs almost all of it. On the side, maybe I also care a little bit about not wasting the energy since I wouldn't be customizing it at all. On the other side, my software compilation experience perfectly matches my desire to ever do it: 100% for Linux, 0% for Android.
Thank you for the link. I went looking for that in the past but gave up, apparently too soon.
https://archive.mozilla.org/pub/mobile/releases/68.0b13/ has the APKs plus signed hashes though.
(But more generally speaking: Why don't you build Firefox from source if you're so concerned with middlemen messing with the code?)