firebirdd-2.1 is a debug version and firebird-2.1 is a relase version.
That is DEFINITELY NOT CORRECT!
firebirdd-X.X is the protocol required for connection to Firebird Embedded while firebird-X.X is the protocol required for connection to the normal Firebird server. The extra 'd' signifies embedded.
I use both protocols & I can assure you that it has nothing to do with debug & release versions.