I agree completely and stand corrected.
What do you think about the cooling effect LN2 would have on a fire? Significant or not so?
One of the other resident firefighters checking in-
RealityChuck put it best - water is cheap, easy to move, and plentiful. How much does 1000 gallons of liquified nitrogen cost? I know that much water (if we had to pay for it to put a fire out) costs about $1.60 in my community. Nitrogen may be plentiful, but not that cheap.
I also have a difficult time figuring out how I would go about deploying a hoseline filled with liquified nitrogen, and fear even more how my firefighters are going to not get the stuff on them. Every agent I have used; water, foam, dry chemical, halon, halotron, dry powder, and CO2 has gotten on either me or a member of my crew. None of the agents listed will cause harm to a firefighter in his/her normal protective clothing (as long as they’re on-air), and we can just wash it off when we get outside. Liquified nitrogen, on the other hand, will cause some significant damage to both my gear and myself.
As for the sprinkler system in the computer server room - it is very possible that the system is a preaction system, not a normal wet system. In a preaction system, the piping is full of air at atmospheric pressure, and the sprinkler heads are closed. If two or more detectors (smoke, heat, or flame) activate, or a single pull station, the preaction valve will open, allowing water into the piping, but not flowing from the heads - essentially turning the preaction system into a wet system. The heads nearest the fire will fuse (like on a normal wet system) from the heat of the fire, and thus dumping the water on the fire.
Preaction systems are becoming quite common in rooms where you don’t want the expense of a clean agent system (halotron, FM200, Inergen, etc), but also don’t want the water damage from a leaking or accidentally opened wet system.
I’m not aware of any studies or other modeling that has been performed on fire behavior and the effecacy of LN2, so while I’d presume it to be an effective coolant, other agents outweigh it when a cost/benefit analysis is done. The inherent hazards also discourage it’s use-we’ve got enough going on that’s ugly with a simple room and contents burn. 
Another Firefighter checking in. LN2 is, as many have pointed out, a cryogenic liquid. The use of it (we’ll just assume there are pumps with the ability to deliver all you’d need to actually fight a typical room-and-contents type of fire) would create a whole new set of hazards both in supression systems and in delivered applications. Imagine fighting a hot room fire, only to burn youself AFTER the fire was put out. It seems that the term “NFPA” is applicable because there is No Fecking Practical Application to LN2 as a common firefighting agent.
Water will continue to be the method of choice for extinguinshing common, AB and C class fires.
Liquid nitrogen has a low heat capacity (~0.00025 kcal/g°K) and low heat of vaporization(0.05 kcal/g). That makes it a poor cooling agent. You can put your hand in a vat of LN[sub]2[/sub], and pull it out without damage.
For the record, I’m not saying that cooling the fuel will always put out the fire, just that it is an effect that will push the fire towards its death. If you have a big enough fire, it is generating a lot of heat, and that can obviously ‘push back’ and cancel out the cooling effect.
The point was simply that low temperatures are a factor, not that they’re a winning trump every time.
Oh no, the dangerous dihydrogen monoxide raises its ugly head again.
Some military aircraft have used Nitrogen (gas) as a fire suppressant in fuel tanks for many years. Perhaps because of a possible vulnerability when under hostile fire.
Also aircraft tyres are commonly inflated with it.
Car tires are routinely inflated with a gaseous mixture composed of 78% gaseous nitrogen.