China's yuan firmed against the dollar on Wednesday, after the central bank set its official guidance at a new seven-month high, but corporate demand for the greenback was seen curbing gains. The People's Bank of China set its daily midpoint higher for the sixth straight trading session to 6.7858 per dollar, its strongest level since November 9. Wednesday's midpoint was 76 pips, or 0.11 percent, firmer than the previous fixing at 6.7934, reflecting broad weakness in the US currency.