Most people don't realize it, but Firefox will do integrated authentication like NTLM (Windows pass-through) just like Internet Explorer. Some people solve the issue by going around Firefox and hosting IE right in Firefox. The other way to do it is to keep Firefox as the rendering engine and tell Firefox it's OK to use integrated authentication with a given site.
The problem is that managing the list of sites you allow Firefox to pass-through authenticate with is not straightforward and involves manually manipulating configuration settings.
This add-on makes it easier to manage this list, allowing you to stick with Firefox but still use pass-through authentication like Windows/NTLM or Kerberos.
NOTE: This add-on does not actually DO the authentication. Firefox itself already has built-in integrated authentication, it's just not obvious how to get it to work. This add-on makes it easy to configure Firefox to use its already-existing features, but it does not do the authentication proper.