Nothing, so long as you're okay with telemetry, bundled junk like pocket, and the removal of the compact UI (see the thread from yesterday - https://news.ycombinator.com/item?id=26464533).

Firefox's internals are great -- it's never been faster or as stable, and for that we made real sacrifices like losing XUL extensions but increasingly I don't understand Mozilla's decision making.

EDIT: I understand many of the things I'm talking about can be fixed with about:config. I had a list of over 30 flags in my notes and it was becoming untenable to patch all my devices whenever a feature I needed was changed or removed. Whereas with Waterfox, I so far have only one about:config change:

    dom.security.https_only_mode = true