By "illegal firearms" I mean firearms that have been stolen, have the SNs filed off, and the like. The Second Amendment doesn't protect trafficking in stolen weapons.
But that's not what Da Mayor means. He means any gun that his thugs haven't given your permission to possess. There are no "illegal firearms", only stolen property. The serial number business is a recent innovation anyway. If the manufactures wish to put them on for warranty or purposes that's fine, but the requirement to have them is a federal law allowing for them to track them. Supposedly if they are found at the scene of a crime or some such. Despite what you see on TV, this rarely happens, and in any event only allows tracking back to the last owner who bought the gun through a FFL dealer. If one or more private party sales occur between that sale and the theft, they can't even get back to the lawful owner.