It cannot come to iOS until Apple allows Gecko to come over. It may happen eventually if Apple ever loosens up more on what engine is allowed besides Safari.
I can't see it helping much unfortunately in bringing more over, since Firefox desktop isn't that big either.
Firefox on Android would be perfect for me, if I could ever figure out why Firefox randomly freezes using the Google search fixer addon from time to time, and if Firefox would ever get tablet support (tabs).
How hard would this be to implement? I feel like even if devs just make the address bar corners rounded to match Material You, that would go along way to making the browser look more polished.