How to Enable Windows Authentication in Firebird 2.5
Few basic things things to check - 


Check user name length. Like in few other databases, this could be failing if the Windows user name length is greater than the permitted limit. From FireBird's documentation this seems to be 31.
Try using Trusted mode and if needed enforce Windows authentication.


Is this a web application connecting which is connecting to this database? If so what's the browser being currently used.