I watched the test firing
I didn't see you in the crowd 
Well, there were about 10,000 people there.
Beside the engine there was a small building with what looked like air conditioning vent lines connected toward the base of the sections slightly forward of the nozzle. What are they and there function?
I don't know, because I've never actually been on the test stand when all the equipment is there. (Nearly all of Orbital ATK's facility is strictly off-limits to anyone but employees at all times.) But I'm pretty sure those are standard portable air-conditioning units. The temperature of various components in the test is a crucial variable, and sitting out in the desert sun will make it hotter than allowed. I don't recall those units being there fore QM-1, which was a hot-temperature test. QM-2 was a cold-temperature test, so they actually cold-soaked the entire booster on the test stand. I'm betting those were there to maintain 40 F temperatures inside the booster after the cold-soak housing was rolled back.
There was also a fire extinguisher rotated into the nozzle at the end of the test, what was burning at that time?
Mostly the remainder of the rocket fuel, but also other materials. Keep in mind that a solid rocket works essentially the same as a liquid-fueled rocket in that it produces Newtonian thrust by constraining an energetic thermodynamic reaction to expand only through a relatively small opening. The solid fuel lines the walls of the steel tube, with a cavity in the middle extending the length of the fuel load -- the "grain."
At ignition, a flamethrower shoots a flame down the entire length of the grain cavity, igniting it all at once. It burns from the center outward, along the length of the grain. In the first few microseconds the expanding gases can just pass easily out the nozzle if they need to without hindrance, but at a certain point a nonlinear response happens when the pressure from burning gases rises produces a gas velocity that makes the flow viscid with respect to the nozzle opening. At that instant the pressure in the casing rises dramatically and within another few milliseconds you have the customary convergent-divergent flow. All that happens in a handful of milliseconds, and during that time all the booster seals must activate.
Okay, so now it's at steady-state, which lasts two minutes. But when happens when the combustion front reaches the casing walls? Ideally the grain burns so evenly that the flame front reaches the casing at exactly the same instant all over the casing. In that case, combustion would abruptly cease. Instead what happens is that some areas of the grain are exhausted first, and combustion peters off gradually. At a certain point the flow once again becomes inviscid; hot gases are still being produced, but they have the huge empty casing to expand into and thus do not create enough pressure and resulting flow to be propulsive.
At this point in the test the quencher moves in and extinguishes the remainder of the fuel. In space, the SRBs will be jettisoned as soon as their thrust decays to the point where the structure can separate safely, and the non-propulsive afterburn happens after staging. You can see this happening in videos of STS staging.
But the smoke that you see at the end is black, indicating that something besides the normal propellant is burning. The propellent isn't filled right up to the steel casing. There's a layer of insulation between the propellant and the casing. So naturally when the propellant is nearly exhausted, the insulation will finally decompose and combust, having served its purpose. But for the test we don't want that stuff burning any longer than necessary in an Earth environment. Especially a Utah environment. Because of some of our unique geology, we often have poor air quality due to a mixture of aerosolized particulates and vehicle exhausts. If you'll pardon a bit of vulgar Western U.S. slang, the past week or so has been "hot as balls," and the ground is very dry. I'd estimate at least half the plume generated by the test was dust blown into the air. (The
test stands aim the plumes against mountainsides.)
