PFB behavior refers to the preciousness that new parents display of their new child's health, safety, development, and general wellbeing.
Example 1.
Ensuring that a person caring for a 3 day old baby knows what they are doing = normal caring parent.
Ensuring that your own good mother, who raised three of you, has read the three A4 page list of instructions before taking your 3 day old hefty healthy baby into the next room while you are at an appointment = PFB behavior.
Example 2.
Ensuring that your new baby's cries are attended to and his needs are properly met = normal caring parent.
Ensuring that you are never more than arms reach from your baby in an otherwise empty secure flat, and refusing to close your eyes unless your partner's eyes are fixed upon the baby's still sleeping form = PFB behavior.
Example 3.
Ensuring all baby equipment is safe to use = normal caring parent.
Ensuring that the moses basket is never further off the floor than the thickness of the carpet in case the handle snaps and the baby falls and and bleeds and dies and you'd be forced to commit immediate suicide with petrol and a match (for extra, well deserved pain to punish you for your rampant stupidity and carelessness) = PFB behavior.
When we refer to a Precious First Born - we don't mean, actually, that the child is more precious, we mean that the child has been subjected to more parental precious behavior.