LONDON (AFP) - World oil prices firmed on Wednesday before the eagerly-awaited weekly snapshot on US energy inventories in leading consumer the United States. Traders meanwhile digested news that the Organization of Petroleum Exporting Countries (OPEC) has lifted its forecast for world oil demand growth. New Yorks main contract, light sweet crude for April delivery, added 32 cents to $81.81. Londons Brent North Sea crude for April delivery rose 33 cents to stand at 80.24 dollars per barrel.