Not only the above but also, ARM doesnt work like that, even with arm mini computers, arm based laptops, and eventually arm based desktops, need a image purposefully designed for the hardware.
You cant boot generic Ubuntu on the raspberry pi, you have to download ubuntus specific raspberry pi image.
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.
@AppLee this your message has to be somwhere on first landing pages. This is not quite good for marketing part of the project, but informative and fair enough for new users.
