Well, they are to an extent with their work on the new Firefox Preview for Android, which is using underlying browser tech as a library instead of bundled together into a monolithic app, which improves code reuse for Focus and Firefox for Android. I don't know if this will translate at all for making separate UX on different platforms, but the optimist in me thinks it will.

That being said, I like the Firefox UI on Linux, Windows, and Android, so I guess I don't understand the desire for making it "more native". My excitement for the new Firefox for Android is all about the performance improvements, not the UI design.