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 (220.127.116.11)
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.