PaleMoon, as Jody more or less covered, is largely for people who don't like the revamped Firefox UI. While I personally wish they'd figure out a way to make more of the page title visible, the rest of the revamp is really not that big a deal. But some people will complain if you move something a single pixel. We used to call them Mac users, but you're starting to find them more and more in other places. As someone who's done some development work, I know there comes a time in any project where it's just faster and easier to chuck a bunch of code rather than try and keep shoehorning new things into it.
But I digress. The two main draws of PaleMoon are that it's 64-bit (which currently means very little, but with WebGL might become a much bigger deal) and it doesn't have the new Firefox interface. Drawbacks would be, in addition to what has already been mentioned, slower updates to fix issues and there's just nothing about a web browser (currently) that needs 64-bit so it's kind of 64-bit for the sake of being 64-bit. You're kind of banking on WebGL not becoming the next VRML (anyone even remember that) or websites becoming so large and complex that the 2GB RAM ceiling placed on 32-bit Windows processes will become an issue any time soon to have any kind of technical justification for it.