The most recent firmware is released for the Fuji X100S. The version number is 1.03. My camera is not old an contains 1.01 from the factory, so I have updated to this firmware. Both 1.02 and 1.03 fix problems I have not experienced – issues with the OVF (optical view finder) brightness at ISO 1600 and a grainy, tv like appearance sometimes on the preview window.
You can find the update here: http://www.fujifilm.com/support/digital_cameras/software/firmware/x/x100s/