Long-term U.S. mortgage rates slipped this week, a sign that borrowing costs are largely stable even as the Federal Reserve has raised its benchmark short-term interest rate. Mortgage buyer Freddie Mac said Thursday that the average rate on 30-year, fixed-rate mortgages slipped to 3.9 percent, from 3.94 percent last week. The rate is higher than it was a year ago, when it was 3.57 percent. But it is still historically low. Before the 2008 recession, it was typically closer to 6 percent.