Thanks to the the-edmeister for giving a solution to this issue https://support.mozilla.org/en-US/questions/1007248
That type of message might be related to Insecure NTLM (pre-NTLMv2) authentication being disabled in Firefox 30.
Type about:config in the URL bar and hit Enter. Accept the warning. Type NTLM in the Search bar at the top. Right-click network.negotiate-auth.allow-insecure-ntlm-v1 and select Toggle. Then close Firefox and restart.
See if you can access that SharePoint website without getting that 401 message.