Something that isn't needed is called by the code - but that doesn't tell why Firefox is calling kerberos in the first place.
The reasons for calling for it would include authentication, and that's it. Authentication to a site that needs kerberos tickets, so that could be e.g. work related sites or similar. But if Firefox calls for kerberos ALSO when it's launching an empty page that doesn't make sense.
Do you have multiple users in your system? Would that same authentication thing happen for other users too?