Why do authenticated forms embedded in unauthenticated web pages display a 401 Access Denied error using Firefox 40?

Version 40 of Firefox intentionally disables authenticated content embedded in unauthenticated web pages if the page and content are in different domains. 

This can be manually fixed by:

• typing "about:config" in the address bar
• clicking "I'll be careful, I promise!"
• searching for the Preference Name:  network.auth.allow-subresource-auth
• Click the "1" next to that in the Value column and change to "2"
• click OK. 

Once you refresh the page it will prompt for login.