For local data, the embedded server of Firebird is very welcome because it does not needs any configuration.
With Firebird 2.5, a special DLL (fbembed.dll) was needed, but Firebird 3.0 has added this functionality into the standard fbclient.dll.
So, if using the embedded server of Firebird 3.0 instead of the one of Firebird 2.5, you need to specify this in the connection string. It is the best to use the ClientLibrary property of the FBConnectionStringBuilder (namespace FirebirdSql.Data.FirebirdClient).
The following files need to be put in your application directory (please make attention to use the correct bitness – 64 bit files for a 64 bit application and 32 bit files for a 32 bit application):
Normally, you should NOT need to put a firebird.conf file in you application directory.