Isn’t that a fork of Firefox that still relies on Firefox development?

Yes. And it seems like a lot of people who shill for it don't understand that. If every Firefox user switched to LibreWolf then there would be no more Firefox and then no more LibreWolf. Firefox has done some questionable things lately but all of us jumping ship to something like LibreWolf isn't the answer.

Would it continue to exist if Mozilla shutdown and Firefox was no longer maintained?

Nope.