I have the late 2013 MBP 15″ with Retina and recently I've been unable to connect to websites not using SSL with either Firefox or Safari. This includes other applications that use HTTP such as the Twitter official app and the App Store.
These sites work fine using the Tor Browser Bundle, and other machines on my network seem to have no issues. This problem persists across other networks and ISPs (Tested with Verizon fios and Time Warner cable.)
If I attempt to traceroute an unencrypted site (for example, honda.com) it ceases to progress after hitting vlan38.dca2a-fdisa-sw2-msfc1.netsrv.digex.net (164.109.3.146)
Edit: Problem persists with both Root account and freshly created non-administrative user account.
Edit2: All proxy settings have been disabled, both in wireless settings and Firefox. Problem persists.
Best Answer
I've fixed this problem awhile ago on a customer's machine, in that instance I believe it was an issue with the WebKit framework.
I'm assuming you don't have access to an external bootable drive, (if you did, boot off it and test). Try booting to your Recovery HD and see if "Get Help Online" works. If it does we know the problem resides in your OS, I recommend backing up your machine and performing an install of your OS, note a clean install should not be necessary unless an archive install fails to resolve the problem.