The card text is very correct on the card mechanics; "Atomic Firefly" has to be face-up BEFORE the attack is declared.

"Atomic Firefly's" effect activates in the Graveyard when "Atomic Firefly" is sent to the Graveyard at the end of the attack, so "Skill Drain" will not negate its effect.

You can use "Barrel Behind the Door" against "Atomic Firefly's" effect.

Barrel Behind the Door: You can activate "Barrel Behind the Door" against Trigger Effects like"Atomic Firefly"

Fox Fire: In order to activate its effect, "Fox Fire" must be face-up when the attack that destroyed it was declared (like "Atomic Firefly"). If a face-down "Fox Fire" is attacked, its effect is not applied even though it is face-up when destroyed.

Giant Kozaky: Like "Atomic Firefly", "Fox Fire", and "Poison Draw Frog", if a face-down "Giant Kozaky" is attacked and destroyed, its effect is not applied. However, if a face-down "Giant Kozaky" is attacked, and is not destroyed by the attack, if "Kozaky" is not face-up on the field, then "Giant Kozaky" is destroyed by its own effect after damage calculation, and damage is inflicted by its effect.