For all the nail-biting about the economy over the past year, investors had it easy: a 5% yield on short-term U.S. debt, one of the safest investments in the world. Now, the times they are a-changin’—again.

U.S. stock index futures edged lower on Friday, pausing after the previous session's rally set Wall Street's main indexes on track for weekly gains following the Federal Reserve's pivotal stance on monetary policy earlier in the week.