One time I did have to press the main deck cargo fire arm and depress/discharge switch due to a FIRE MAIN DECK AFT message. Unfortunately this was at about 30 west on the NATs. Not a good feeling. The depressurization is fairly controlled and the remaining operative pack did a fine job of maintaining cockpit temperature both during the descent form the 30's and at FL250. By the time we arrived at the nearest airport it was apparent that the fire warning was a smoke detector system malfunction, so the approach was relatively normal. An ACM on portable O2 was dispatched to the main deck to investigate and fight the fire if necessary. The grim prospect of ditching was of course going through our minds had the fire been real. A few "monday morning quarterbacks" claimed we should've have disregarded the warning due to the slim likelihood of a cargo fire igniting 4+ hours after takeoff. I think you have to take the warning seriously and o everything you can to get on the ground ASAP. Had we ignored the warning and it turned out to be real, we would've been posthumously ridiculed as muppets.