On most DV camcorders the firewire is active when in the Camera Mode allowing the cam to be used as a camera head whether or not tape is rolling, one catch is if on battery without tape rolling there is usually a timeout(approx 7min) to reduce battery drain so its best to use AC power or have tape rolling, or the transport door open

Since the firewire is always carrying a signal in the Camera Mode you can do a manual capture into a computer if the software is setup right, you would also have to start the tape manually, I've also used the firewire output in the Camera Mode to record the signal with a small deck like a video walkman while changing tapes to provide continuity on long shoots