5m is 15ft. But keep in mind, these standards are beautiful, but are only "safe" values. this doesn't mean it will work at the given value nor that it can't work over a certain value. best bet is just give it a shot and hope it works.
I don't know for firewire, but pretty much all other computer based cables can be stretched the length of a vehicle (15ft or so) without a problem as long as they are shielded. Only exception would go to regulated 12v power cables and antennas which should be kept to a minimum to keep interference out.