Ever wondered how to make Firebird talk to MySQL, PostgreSQL, or any other database through ODBC? Denis Simonov has crafted an excellent deep-dive into creating custom Provider plugins for Firebird that does exactly that:

Writing custom providers for Firebird

In this article you will find the coverage of implementation challenges encountered during the development of HQbird's MySQLEngine and ODBCEngine plugins, including architectural solutions for DBMS limitations.