Firewall is a Continuous Trap Card.
Trap Cards are normally Set first and can be activated on a later turn, often in response to an action.
On the current TCG and OCG Forbidden & Limited lists tracked here, Firewall is Unlimited.
YGOPRODeck lists Firewall for these formats: Speed Duel, Edison, Common Charity, TCG, OCG, Master Duel.
A listed print of Firewall appears in Force of the Breaker (FOTB-EN060) at Rare rarity.
Among markets shown on this page, the lowest listed snapshot for Firewall is about $0.07 on Cardmarket (prices change frequently).
