This is something you are more likely to come across in an Intranet environment, but it is a useful trick configuration to know about. There are 3 settings you need to change in Firefox in order for it to work. Here’s how to do it:
In the address bar, type about:config
You may get a warning message, this is fine.
From the list produced, find the following 3 properties:
You need to add the server you wish to authenticate to the end of the value associated with the property, so it should read something like “localhost, myserver.com”.
You need to set that for each of the 3 properties and you should be good to go, no restart needed.
Like I said, you are unlikely to need this in an Internet environment, but it is necessary to authenticate with Windows authentication in an Intranet environment.