Possible Clues:

• As it can't be bettered, so it turns out it can't be fired
• One can't get fired in the way of the 3 down rounds
• Like the best circles, cannot get fired
• Material decrees to restrict yobs swamping the east of France
• Old building substance
• Litigation-prompting mineral
• You can't make light of it
• Like what's very good, so one is back, never to get fired
• Fire-resistant material, injurious to health
• Fiberglass predecessor