Is there a graphical tool to design a Firebird database using diagrams?
You are probably looking for some kind of CASE tool to show database tables as diagram. Many commercial GUI administration tools have this option. There is also a very good freeware IBUtils tool by Ales Kahanek:
It is able to import the tables from the existing database and display the relations (foreign keys). You can select a subset of tables for each model. It also supports printing and exporting as JPG, WMF or BMP images. You can also change the tables withing the program.