We've got a new MacBook Air which has a new to us problem. It's unable to web browse to many common websites which normally auto-escalate from HTTP to HTTPS. Example, one can normally go to trello.com or pinterest.com just by that name in the browser and it'll auto bounce up to HTTPS. With this new machine it fails to connect. However, if you manually do https in the URL bar, it will connect.
Notably it doesn't affect all websites (google.com is fine, among others). It's also limited to this specific machine. Our old Macs don't exhibit the same behavior. OS X is fully up to date. I've even tested with Firefox and the problem happens there too, so it appears to be OS level (although, above ICMP level as pings work perfectly fine of course).