I've used Firefox heavily at work ever since they built NTLM authentication in, but have always been rather annoyed at how limited it is compared to the support built into IE. With IE there is no difference between accessing an intranet site that requires NTLM, and one that requires no authentication, assuming you're already logged into the network on your machine. Firefox on the other hand, would bring up its authentication dialog for each site, prompting for a username and password.