It's been a minute since I've used Librewolf so maybe things changed, but I remember that not being able to sync between devices was the biggest difference. There wasn't an account login option in LW, but there is in Waterfox.
Other than that, I dont remember any real differences. Waterfox has been fine for me because of the sync, but if you don't need account syncing, LW is fine.
edit: oops, there *is* a LibreWolf Mac port now