He is also a chess writer, most famous for his work in writing the 13th, 14th, and 15th editions of the important chess opening treatise Modern Chess Openings. He has represented the United States at several Interzonals and played on the United States Olympiad teams of 1980, 1984, 1986, 1988, 1990, 1996, 1998, and 2000. De Firmian earned the International Master title in 1979 and the General Motors title in 1985.
He has a degree in physics from the University of California, Berkeley.
Grandmaster de Firmian is a noted expert on the chess openings and in 1990 he revised Modern Chess Openings, 13th edition (MCO-13). In 1999 he wrote the 14th edition of Modern Chess Openings (MCO-14), which, along with Nunn"s Chess Openings (NCO), is considered an outstanding single volume opening reference in English.
He also helped prepare the chess opening book for the International Business Machines Corporation Deep Blue team for its successful 1997 match with Garry Kasparov. In 2006 he revised and expanded the classic 1921 book Chess Fundamentals, by José Capablanca.
The edition was harshly criticized by chess historian Edward Winter, who claimed that de Firmian "destroyed" the book by changing Capablanca"s writing and removing games from previous editions to include new games not played by Capablanca.
De Firmian also wrote the 15th edition of MCO, which was published in April 2008.