Jan 6 (Reuters) - Copper prices fell on Thursday as the dollar firmed after minutes from the U.S. Federal Reserve's meeting signalled that the central bank may raise interest rates sooner than expected. While a firmer U.S. dollar makes greenback-priced metals more expensive to holders of other currencies, an interest rate hike could trim liquidity in financial markets and slow recovery in the world's biggest economy. * Asian shares fell on Thursday, extending a global slump after Federal Reserve meeting minutes pointed to a faster-than-expected rise in U.S. interest rates due to concerns about persistent inflation.