I am trying to fix a friend's Macbook (13-inc, mid-2010) which is running OS X 10.11.6 (El Capitan). Safari and Chrome open and the application is responsive, but trying to load a web page gives the rainbow wheel of doom. The App Store also won't load - it's just the grey screen. I thought maybe it was a network issue, but oddly enough Firefox works just fine - no problems browsing the internet.
So then I thought maybe it was a WebKit issue, since Safari and Chrome both use WebKit as the rendering engine, whereas Firefox uses Gecko. I think the App Store also uses WebKit, but I'm not entirely sure.
If I boot in safe mode, Chrome and the App Store work fine, whereas Safari won't open at all. But given that Chrome and the App Store work, it makes me think there's something wrong at the system software level, but I'm not sure what. Googling WebKit OS X a million ways didn't lead me to any useful resources.
What could cause Safari, Chrome and the App Store to have issues loading? Is it a WebKit issue? If so, what steps can I take to reconfigure the system to resolve the problem?