ZURICH, Aug 1 - The Swiss franc opened firmer ahead of crucial U.S. jobs and July manufacturing data on Friday, expected to underscore that the world's business engine is on the mend. Trading was set to stay thin with many dealers away from their desks for the Swiss national holiday. The dollar eased to 1.3710/15 francs from 1.3741/46 late on Thursday in Europe after sharp gains on upbeat U.S. economic indicators and more weak numbers from Switzerland.