To be fair, Ubuntu touch is something I guess will die sooner or later (my opinion, but their community can show me I’m wrong about). The current Linux phone development is much closer to build around the already existing things from desktop. I run native LibreWolf as main browser (Firefox ESR was preinstalled), which was supported since day one. Everything that can run on a RasPi can also run on pocket computers (ARM CPU architecture can limit in the same way as on RasPi).
There are just some CSS hacks required to run Firefox smooth, since Mozilla does not care about little screens. For common users it’s a download and packagers do also include some of these hacks as default installation on newer OS versions.