Jenny Chang-Rodriguez/BIThe economy is about to see net job losses as firings pick up, David Rosenberg says.The economist says it's only a matter of time before layoffs exceed new hires.He has predicted the unemployment rate will jump past 5% by the end of the year.The US could start to see negative employment reports rolling in by year-end.That's according to David Rosenberg, a top economist who's been warning of weakness in the US labor market for months.The Rosenberg Research founder foresees the unemployment rate rising past 5% by the end of the year.The number of new hires has slumped for most of the past year.Federal Reserve"The one thing that I'm focusing on that's telling me that we're going to start to see net job loss — I can't predict the exact month — is that I'm watching the hiring rate.