There are lot of third party tools to design, manage and develop Firebird databases. You can easily find them in Google or in the
Firebird tools catalog at IBPhoenix.com.
Under Firebird umbrella there is
FlameRobin open-source project. FlameRobin is a basic database administration tool for Firebird; it's cross-platform (Linux, Windows, MacOS) and dependent only on other open source software.