This issue will occur on a Mac when trying to log into SharePoint using Firefox v30.
As part of Firefox 30′s release, Mozilla made a change to disable support for NT LAN Manager version 1 (NTLMv1) network authentication. This change affects sites using Microsoft’s SharePoint or IIS services. The Windows version of Firefox 30 should switch to using NTLMv2 authentication automatically, but NTLMv2 is not supported by Firefox on non-Windows (for example Mac and Linux) platforms.
This will result in getting a 401 UNAUTHORIZED, 403 UNAUTHORIZED or 404 UNAUTHORIZED error
Mozilla has provided a workaround to non-Windows users of Firefox, in the form of a setting that can be toggled to allow NTLMv1 authentication. This workaround should allow Mac and Linux users to continue using NTLMv1 authentication, which will allow access again to SharePoint-based or IIS-backed web applications. For more details, see below the jump.
Enabling NTLMv1 in Firefox 30
- On your Mac, open Firefox
- In the address bar, enter the following command: about:config
- If prompted, click on the I’ll be careful, I promise! button
- In the search box, search for the following: network.negotiate-auth.allow-insecure-ntlm-v1
- Once the network.negotiate-auth.allow-insecure-ntlm-v1 setting is located, double-click on the setting. This will change the entry in the Value column from false to true.
- Once the setting has been set to "true", close the Firefox browser window
- Open a new browser window and attempt to access the SharePoint-based or IIS-backed site again. You should now be able to log in now
Information shamelessly stolen from here.