Does it still apply if the target is paralyzed (or knocked out in general)?

Or can I use quickened metamagic to apply Hold Person, then charge in and blast the guy in the face with a critical firebolt without getting disadvantage?

If the enemy is incapacitated, you do not get disadvantage for making a ranged attack in close combat.

You can see from the below quotes that you do not get disadvantage - you in fact get advantage.

Hold Person paralyses the target which inflicts incapacitated which disables the disadvantage from close combat ranged attacks (as long as they are the only person in melee range with you) - and because they are paralysed you get advantage on them generally and automatically critical if you do hit within 5 ft.

Ranged Attacks in Close Combat (https://www.dndbeyond.com/sources/basic-rules/combat#RangedAttacksinCloseCombat)
Aiming a ranged attack is more difficult when a foe is next to you. When you make a ranged attack with a weapon, a spell, or some other means, you have disadvantage on the attack roll if you are within 5 feet of a hostile creature who can see you and who isn't incapacitated.

Paralyzed (https://www.dndbeyond.com/sources/basic-rules/appendix-a-conditions#Paralyzed)

• A paralyzed creature is incapacitated (see the condition) and can't move or speak.
• The creature automatically fails Strength and Dexterity saving throws. Attack rolls against the creature have advantage.
• Any attack that hits the creature is a critical hit if the attacker is within 5 feet of the creature.

Strategically powerful and very cinematic. I like it.

Keep in mind, that the rule you're talking about regarding disadvantage on ranged attacks isn't about distance to the target. Disadvantage is imposed if any hostile creature is within 5' and isn't incapacitated. The rule does apply if you're shooting a bow from point-blank range, but it also applies if you're trying to shoot at someone 30' away when there's another enemy next to you.