Any fork will lack behind in updates, that includes security updates. This can really affect your security when a zero-day vulnerability is discovered and the development team hasn’t pushed the latest patch yet.

Does that mean that you shouldn’t use FF forks? Not at all. Firefox is neither the most private, nor secure web browser out-of-the-box, anyone claiming such a thing does not know what they’re talking about.

Recommending Librewolf to someone that wants to take their privacy seriously is a great idea in my opinion, but claiming that it is more secure than Firefox is sort of misleading.