When connecting to a web site with Firefox on a Mac you see the following error in the browser's window:
Proxy Authentication Required
This server could not verify that you are authorized to access the document requested. Either you supplied the wrong credentials (e.g., bad password), or your browser doesn't understand how to supply the credentials required.
First seen in
Sophos Web Appliance WS5000
The issue is with Firefox. Firefox is not correctly corrected to use Kerberos for SSO.
What To Do
You need to configure Firefox to use Kerberos for SSO. The article linked below (redhat.com) is provided as an example of what is required.
Note: During troubleshooting it was determined that if Safari is launched first, Firefox is able to browse successfully. If Firefox is launched first with no Safari then browsing fails with auth error. It appears that Firefox is not able to perform the kerberos auth like the Safari browser and this appears to be a Firefox issue.