Background info on Pale Moon from my perspective:
- Started out as plain x64 fork when mozilla stubbornly refused to cover that market for years. Wasn't even the first.
- The author started rejecting some of mozilla's decisions some way back, dropping FF's then new Australis GUI from PM, or setting defaults (like SSL/TLS) more securely. This quickly led to him changing some internal GUID to make the two products distinct on the API level, leading to a major Add-on compatibility break. This is mostly over by now, the better addons adopting PM in their manifests, the rest being rightfully ignored by the PM community... And now PM being the natural haven for addos that are being broken by moz's breaking API overhauls.
- Don't underestimate the subtle security bonus. My memory fails for specifics, but there have been at least two serious holes in FF since v25 that never affected PM simply because its author was proactive in shutting down the enabling outdated cryptociphers or such-so. Or look for the Cookie setting to accept first-party ones but not third-party ones: Gone from FF, available in PM. Or...