Firefox on iOS is Webkit wrapped in a Firefox UI shell. It does not use Gecko, and so is very different from desktop and Android Firefox, much more like Safari actually.
And the fullscreen mode of Firefox doesn't seem to be very well integrated to me. It's the only feature that is still superior with Chrome. I wish there was a way to maximise screen space used by the page displayed itself.
Apple has demonstrated that privacy is a very effective value proposition. Maybe Apple should look into investing in Mozilla/Firefox.