Yes, you need CFS 3 for Firepower. I just got the Aircraft Powerpack, which has Firepower and Wings of Power. It has 2 seperate installs (the powerpack) one for Wings of Power( for FSX/2004) and one for Firepower( for CFS3)
That's funny you mentioned B-17's. Was on a bombing run and tangled with a bunch of Me109's. I like the aspect of being able to switch from various positions on the bombers. I also just added to CFS3 the BOB addition. It's great add on, I like flying that Stuka.