It is my belief that the old combi aircraft are grandfathered, but new types are much more strict. They are known as a Class C cargo area having fire detection and protection. Halon is used in the cargo area which is isolated from the passenger area which is theoretically sealed off from the cargo area.
I suppose it could be different from type to type but in my experience on a 727-200, in the event of a fire, a handle in the cockpit is activated closing off airflow to the cargo area. There are 9 Large Halon bottles. The number activated in the event of a fire depends on what combi configuration the aircraft has at the time as selected on the F/E panel(can range from 4 to 9 cargo pallets along with all cargo or all pax).
Halon fire extinguishers are activated which give a 5% concentration for initial knockdown. The remaining bottles discharge at a predetermined rate to maintain at least a 3% concentration for the remainder of the 75 minute suppression period(60 minutes to landing + 15 minute reserve). No fire suppression is available in all-cargo or all pax configuration.
In the all-freight configuration, the aircraft is thousands of pounds heavier than a similarly equipped aircraft although not all of this extra weight is due to the firefighting equipment.