Firebird 2.5. Windows 8.1. Can't connect to any database! Why?
If you start Firebird as an application, then Firebird must either be located in a normal folder (not a system folder under UAC protection like Program Files), or you need to run it as Administrator.

The reason is that Firebird needs write access to several files in its own program folder, which doesn't work without privilege elevation. 

Better yet: run it as a service. The service by default has sufficient rights to start Firebird.