If you develop Web sites on your local machine and not on a remote server then you want to test them. For that, it is used the URL of your local Web server (IIS, Apache, Tomcat) which is http://localhost.
When using Microsoft IE there is no problem but for Firefox, because some security issues, the browser requires authentication and asks for an user name and password when trying to open Web pages from localhost. The solution for this problem is:
- In Firefox addressbar type about:config;
- Find the preference named network.automatic-ntlm-auth.trusted-uris
- Doubleclick and type localhost
- Save the value
- Test your Web page