There is no separate CRG FIRE warning, only SMOKE. Therefore one has no idea if there is infact a fire burning, or if the first bottle (60 sec discharge)did it's job. Some smoke in the cabin might be a sign, but depends we're the burning article was in relation to the ventilation system/smoke detector location. If a burning object was right by the detector, then the vent would probably be shut before smoke even reached it. If my arse was on fire, I'd throw them down the slides rather then face a possible British Airtours scenario.
It would be interesting to see what the article was. Let's hope something one-off -ish, because if it's one of these lithium ion batteries......