Excellent find, Click! Very interesting. Deserves a sticky.
"F" might stand for France and means the usual obnoxious volume limit imposed by French laws. Installing a "F" firmware starts the player with volume option at "normal", not "high". (Probably this is since they can't just limit the volume when choosing French from the language selections list - Canucks would run amucks.
"P" might stand for Pacific, as I've seen from other Sandisk firmwares. (Optimized for Reggae and Calypso music?)
The other question is - what's with the Clips sold in Europe that have a radio (and cost EUR 5 more)? Do they run on the US firmware?