Is there some free visual query builder for Firebird?
Some standard tools like FlameRobin offer ability to drag and drop tables and columns into SQL editor and it creates joins automatically based on Foreign Key information.
However, there are specialized tools for this purpose as well. One example would be SQLeo Visual Query Builder, which is an open source application that supports many databases, including Firebird:
There is also a freeware from Fast Reports Inc.
It can be used with Delphi and Borland C++ Builder