Pale Moon actually entails in-house development, and incorporates a number of features not present in Firefox ESR. Pale Moon is not a simple rebuild any more; it includes a number of UI changes and additional features (such as TLS 1.2 and OCSP stapling); these are not present in Firefox ESR. Pale Moon is not just about removing features arbitrarily: features are removed if they are not used widely and if they give a sufficient performance advantage.
In fact, the development of Pale Moon and Firefox are completely independent. While Pale Moon bases itself on the Firefox codebase, it will evolve according to its own development roadmap. Some people keep claiming that Pale Moon will jump on to whatever (ESR) version that is released:
Pale Moon intends to become a real fork, in fact, already has a number of major changes. Pale Moon will retain the old UI, no matter what. Work may be involved to keep it free from the Australis UI, but it can be done.