What is Reddit's opinion of Firebird?

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Not sure what you are using but with MS Office products you can do a pass-thru query in the native language of the RDBMS.

Looking at the Firebird reference page I see systems tables:

http://firebirdsql.org/file/documentation/reference_manuals/fblangref25-en/html/fblangref25.html

Hopefully the relations are filled out and then it is easy to construct a schema. If you have Vision Professional it has a reverse engineering feature via an ODBC connection. I would check the documentation for Visio Pro and make sure it can reverse engineer Firebird before you purchase it.