Follow this steps:
1) Open the FireFox browser
2) Type the following `about:config`
3) Look for `network.http.phishy-userpass-length` if you don't find then create a new Integer key
Create a new Integer key (right-click->New->Integer): `network.http.phishy-userpass-length` with value `255`
You need to create a Firefox driver with the following:
FirefoxProfile profile = new FirefoxProfile();
profile.SetPreference("network.http.phishy-userpass-length", 255);
profile.SetPreference("network.automatic-ntlm-auth.trusted-uris", "YOUR HOST ADDRESS HERE");
_driver = new FirefoxDriver(profile);