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A fire line is something that feeds the fire protection system. The wet sprinkler line only feeds the sprinklers. The fire line can feed other fire protection equipment such as hoses. Fire line is usually larger as well because of this.
If it only smells when the furnace is running I would tend to think the gas leak would be on the manifold side of the furnace (gas valve-to-burners). So a pressure test won't really do any good. I wouldn't mess around with a gas leak though. Call a heating company and get someone out there to fix it. Money spent, but you get to sleep safely. cheers.