After upgrading to Mac OS X 10.8 Mountain Lion, I started to experience problems with Safari 6 and some HTTPS connections. For example I could no longer log into Amazon or even browse forums who used SSL connections. Ironically, I found a post on Apple’s forums that described some of my symptoms, but since the support forums are HTTPS, I could not use Safari. Luckily Firefox still worked. The problem on Apple’s forum went on about SSL Certificate issues and the solution is described on this blog posting, but this problem was specific to Mac OS X 10.7.4 Lion. There is also a bug that has to do with specifying a proxy in Mountain Lion. This seemed more plausible to me, since I use pfSense with Squid Proxy in transparent mode at home, however this also would not explain why only SSL connections had issues and regular HTTP sites worked fine.
After much research, it seems the simplest solutions work best. I had to manually specify my MTU setting from 1500 to 1492 in System Preferences – Network – Advanced… – Hardware – MTU. This immediately resolved my Amazon logging in issue.