Go to your Firebird installation directory (C:\ProgramFiles\Firebird\Firebird_1_5 ?), then to the bin\ subdirectory. You'll find a program named isql.exe there, run it. You'll see a SQL console.
I assume you're connecting to local server. To connect issue a command:

SQL> CONNECT localhost:c:\path\to\your\base.gdb
SQL> user 'username' pass 'password';

Since you installed Firebird and probably haven't changed anything use 'SYSDBA' as username and 'masterkey' as password.

After connection you'll see info line:
Database:  localhost:c:\path\to\your\base.gdb, User: SYSDBA
which means you may work with the database.

Hope this helps
Michael