im assuming firewire is running at 5v, as i dont know much about it. but if that is correct, then plugging it right into the car is going to do bad stuff right from the start. but.. you can use a voltage regulator to overcome this AND the varying power, all in one hit. get a 5v regulator, they should be pretty common at any electronics store, or if you cant find one, theres one in every snes, every genesis, and probably scores of other consoles and home electronics. regulators look just like the large transistors, but mostly start with LM if you read the labeling, and they often have i G O printed on the pcb at the base.. theyre black rectangles with a hole in the top (the top is often metal) to screw it to a heat sink, they have three pins, the pins are as follows from left to right look at the front with the pins at the bottom. In, Ground, Out. in is your ~10-15v from the car, ground is, well, ground (or negative if thats how you want it labled) and out is your now safe 5v.
having said all that, you might want to check that firewire is indeed 5v, and not 3.3v. getting 3.3v is harder, youll probably need an adjustable voltage regulator, which will probably have a different pinout. if they are 3.3, post back here and ill tell you the parts to get a 3.3v regulator setup going. (these should be standard.. i dont know why they arent.. then again, maybe they are and i just dont know about it)