As of 2002, France along with several European nations changed its currency from the franc to the euro. If you still have francs as bills not coins from previous trips, you can still exchange them at the Banque de France until February 17, 2012. The franc-to-euro conversion rate is fixed at 1 FF = 0.152449 €. As of February 17, 2005, the Banque de France no longer exchanges coins.
The US Dollar is very volatile against the British Pound, and also the Euro. As the Euro is stronger than the US$, I have included a link to a real time conversion that will allow you to see how many dollars a Euro will buy.