Where is firewall side of the engine?

There's no part of an engine called a "firewall side". The term "firewall" refers to the fire-resistant barrier separating the engine compartment from the passenger compartment of a vehicle. Therefore, the engine is *on* the firewall side, but the firewall itself isn't *part* of the engine. Components of the engine might be *mounted* to or *near* the firewall, depending on the vehicle's design.