I have an early S/N F6 and the big change was battery indication on the LCD. The firmware made the indicators more accurate somehow. I've never had an issue, but I usually us EN-EL4 batteries, so I have tremendous battery life (far more frames then film I'd carry for even extended shooting).
I still have to get my F6 to Melville to get the firmware upgraded, but just haven't bothered yet.
JCA