Even if your application is intended to work with Firebird in the Embedded mode, it is advisable to attach to the full server during development. The reason is that embedded Firebird runs in the same address space as the application and any application connecting to a database in embedded mode must be able to obtain exclusive access to that database. Once that connection succeeds, no other embedded connections are possible. When you are connected to your database in the Delphi IDE, the established connection is in Delphi’s application space, thus preventing your application from being run successfully from the IDE.