I have another Mac OS el capitain workstation which has the exact same problem. To correct the problem I completely uninstalled and reinstalled Firefox.

I originally posted "web site viewable" and marked it as solved because I though it was a DNS issue. That was wrong. The problem is with Firefox. The web site "http://www.flightofthephantom.com/what-is-an-imu" cannot be accessed when using a specific DNS FreeDNS. It however works when I use my ISP's DNS or Google or Quad9. I know this is a problem with firefox because 1. Safari on the same workstation works for all DNS's 2. I created a new user account on the same machine and Firefox works in that new account for all DNS's I have another Mac OS el capitain workstation which has the exact same problem. To correct the problem I completely uninstalled and reinstalled Firefox. I still have Firefox on the original machine with this problem and I would like to know what is the corruption. I have performed the following to correct the issue but was unsuccessful. 1. Refresh Firefox 2. Cleared Cached ,history and cookies 3. I have reinstalled firefox What components remain after the above that would prevent operation of a specific web site (I believe there are other sites) via a specific DNS?