Error: "cannot parse response" (NSURLErrorDomain:-1017)

I see this was placed on the support board a couple days ago and the solution helped identify where the problem lie, but I've been able to go no further. I need help.

So, Last night, running Yosemite, I did the latest security update 2016-001, and all of a sudden, some websites stopped loading after the update. I was using Firefox 43.0.4, but I'm able to recreate this in Safari as well. I know the update caused the problem because the site I was going to was working before the update and then not after. Specifically, goutletpens.com And the site loads on my iPad and every Windows machine I have, but not my main Mac anymore. When I ran it on Firefox, I get a blank page in response. When I run it in Safari, I get the error "cannot parse response" (NSURLErrorDomain:-1017). I would have ignored this, except, working today, I had a similar problem where many other pages I needed, but not all, stopped working or did not work properly either. I had been meaning to update to El Capitan, so I thought I would do that and see if it solved the problem. It did not. I also made sure Java and Flash were updated to the latest versions, just in case. Looking through the support pages, I saw someone else had this problem, and I saw the suggestion to try booting into safe mode and see if the page worked. I did, and everything loaded fine.

So my question is, obviously something that safe mode disabled is causing the problem. How do I go about pulling up the list of potential culprets and disabling them one by one until I discover what got broken the last couple days. I can't just go into Firefox, because I am getting this problem with both browsers.

Thanks.