Starting with which generation of iPod Nano was FireWire charging functionality no longer available?
The third generation began including a 30-pin dock connector, allowing for FireWire or USB connectivity. This provided better compatibility with non-Apple machines, as most of them did not have FireWire ports at the time. Eventually Apple began shipping iPods with USB cables instead of FireWire, although the latter was available separately. As of the first-generation iPod Nano and the fifth-generation iPod Classic, Apple discontinued using FireWire for data transfer (while still allowing for use of FireWire to charge the device) in an attempt to reduce cost and form factor. As of the second-generation iPod Touch and the fourth-generation iPod Nano, FireWire charging ability has been removed. The second-, third-, and fourth-generation iPod Shuffle uses a single 3.5 mm minijack phone connector which acts as both a headphone jack and a data port for the dock.