This error is caused by a known issue with Firefox, Localhost and IPv6 settings.

There are three possible solutions to this issue and any one of them should solve the issue:

1. Make a copy of the local client shortcut, access the properties and change the URL from http://localhost:8400/Default.aspx?hidebrowsermenu=true to http://127.0.0.1:8400/Default.aspx?hidebrowsermenu=true.  This will bypass the localhost issue and allow the login page to display quickly.
2. Instead of using the shortcut, launch Firefox and point to http://127.0.0.1:8400 and the login page will display immediately.
3. You can disable the IPv6 setting in Firefox through the following steps:
4. Launch Firefox and type “about:config” in the address bar.
5. If prompted with a warning, choose accept.
6. In the filter, enter “IPv6” and it should display one entry named “network.dns.disableIPv6” which is set to false by default.
7. Right-click network.dns.disableIPv6 and select Toggle, this should change the value to true.
8. Now the login page should display immediately (you may have to allow pop-ups first) using either localhost or 127.0.0.1, and when using the default shortcut.