For several years, a man has held a single, dominating belief that he was wrongly dismissed from a job, a belief with a plausible basis in reality, held with strong conviction, though he is somewhat more willing than most to discuss and consider other perspectives on it than a typical fixed false belief would allow. What is this?

This is an overvalued idea: a single idea or belief that dominates the person's thinking, often for years, usually with some basis in reality, held with strong conviction but with somewhat greater openness to outside opinion than a true delusion. It is not a delusion, since it differs from one primarily by degree — an overvalued idea has a more gradual onset, more plausible content, and greater openness to discussion, and can occur in people with no diagnosable mental illness, unlike a delusion. It is not an obsession, since he is not resisting this belief as unwanted or irrational — he holds it with conviction rather than experiencing it as an unwelcome intrusion he tries to dismiss. And it is not a primary delusion specifically, since that requires a fixed, culturally out-of-keeping belief that cannot be traced to any understandable origin, which does not fit a belief openly acknowledged to be about a real, plausible event.