Note that out of these, the PaleMoon browser ( https://www.palemoon.org ) is the only real "hard" fork of Firefox. The others are all "soft" forks of Mozilla Firefox in that they all just customise some existing settings as defaults or customise the UI or integrate their own extensions of Firefox and rebrand it.
The PaleMoon team however forked even Mozilla Gecko ( https://en.wikipedia.org/wiki/Gecko_(software) ), the browser engine that is at the heart of Firefox to create the Goanna browser engine ( https://en.wikipedia.org/wiki/Goanna_(software) ) that now powers the PaleMoon browser and the Unified XUL Platform (a framework that can be used to create multi-platform desktop applications with web technologies).
Another "hard" fork of Firefox Gecko is the Servo browsing engine ( https://servo.org/ ) though no browser application has been built on it yet.
The PaleMoon team however forked even Mozilla Gecko ( https://en.wikipedia.org/wiki/Gecko_(software) ), the browser engine that is at the heart of Firefox to create the Goanna browser engine ( https://en.wikipedia.org/wiki/Goanna_(software) ) that now powers the PaleMoon browser and the Unified XUL Platform (a framework that can be used to create multi-platform desktop applications with web technologies).
Another "hard" fork of Firefox Gecko is the Servo browsing engine ( https://servo.org/ ) though no browser application has been built on it yet.