First of all, the UI of Firefox isn't responsive or convergent so it's barely usable on small screens. Other distros on the Pinephone/Pinetab has some workarounds but it's still not as usable as the Morph browser on smaller screens. Another factor is that on android-based UT devices, pure Mir is used which Firefox does not support and probably will never do. However, UT will be transitioning to wayland on Mir, which the Pinephone/Pinetab already use, and once that's fully done then I would think it'll be easier to user Firefox on UT with decent performance. At the moment, it'll be slow on Libertine/Xmir.