Hopper for Firebird is a standalone tool to debug database stored code, like Stored Procedures and Triggers. These routines normally run at the server, inside your database and cannot be debugged with your normal development tool.
Stored Procedures, Functions, and Triggers are often used during database development. Firebird don't provide a way to debug stored code in the database running at the server nor provide a programmers API to create one.
Hopper fills this gap by emulating the server side code at the client computer allowing you to step through your code statement by statement. This gives you the much needed insight in your code during development and bug fixing.