Firestone received his Ph.D. in clinical psychology from the University of Denver. In his doctoral dissertation, A Concept of the Schizophrenic Process (1957), Firestone postulated that schizophrenics rely on internal, fantasized "self-mothering" as a replacement for secure external interpersonal relationships. He later expanded and generalized this line of thought to describe what he called the "core defense" (for non-schizophrenics) in his theory of "the fantasy bond."Firestone also attended University of Syracuse and in 1951 graduated from the University of Berkley.
Firestone worked as a clinical psychologist in private practice from 1957 to 1979. Since 1979, Firestone has been affiliated with the Glendon Association. Firestone is also a contributing psychologist to www.PsychAlive.org. Firestone developed the Firestone Assessment of Self-destructive Thoughts (FAST), a scale that assesses suicide potential, the Firestone Assessment of Violent Thoughts (FAVT), a test that examines violent thoughts that can lead to violent behaviors, and the Firestone Assessment of Suicidal Intent (FASI), which examines how a person's internal dialogue puts them at risk for suicidal behaviors.