Pale Moon is a 3.5 fork, it's ancient, although it's gotten a lot of independent development.

AFAIK it's rebased onto Firefox 38 and there will be another rebase in the future based on Basilisk release (should be similar to Firefox 53-54)

Do you not wonder that this fork will suffer the same fate?

Waterfox has been around for a while. What fate are you talking about? Pale Moon is still in active development and use as well. I use it on some older machines for performance reasons.

Because Mac?

Also, Pale Moon forked much longer ago and has diverged more significantly from Firefox, AFAICT.

when the first firefox 57 beta came out I tried pale moon and too many of the extensions I used wouldn't work on it. They worked with Waterfox. I use firefox as my main browser but there are a couple extensions that won't be ported over that I keep waterfox installed to use.