He has represented the United States at several Interzonals and played on the United States Olympiad teams of 1980, 1984 and 1986. De Firmian earned the International Master title in 1979 and the GM title in 1985. He currently resides in Denmark with his wife, Christine, who is a chess expert and past member of the Danish Women's Chess Team.
He won the 1983 Canadian Open Chess Championship. In 1986, he won the World Open and the first prize of $21,000, at that time a record for a Swiss system tournament. De Firmian was a founding member of Prochess, a grandmaster advocacy group dedicated to promoting chess in the United States. 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 IBM Deep Blue team for its 1997 match with Garry Kasparov.
In 2006 he revised and expanded the classic 1921 book Chess Fundamentals, by José Capablanca. He has also written the 15th edition of Modern Chess Openings, which is scheduled for publication in April 2008.