If I remember correctly, ~10 years ago one of Waterfox's main points was 64-bit + Intel compiler with aggressive optimization flags. At that time Firefox was 32-bit only on Windows, at least the stable version. From my experience, it didn't feel faster. Actually, it felt slightly more sluggish, I guess pointers and some data structures become larger, so the browser used more memory and put more pressure on CPU caches. It's probably no longer relevant as all major browsers are 64-bit by default.

Yeah, kind of the same. Out of the other "firefox" clones, Waterfox is my goto.