In Amadeus, Salieri presents himself as a conventionally well-behaved man who wanted to excel as a musician. This was his attitude toward his own creativity, his successful musical career, and his treatment of other people. But once Salieri meets Mozart, all these ideas are cast into doubt. Mozart does not embody goodness—Salieri finds him an “obscene child”—and Salieri's jealousy of the young genius turns him into a monomaniac. Salieri starts behaving far worse than Mozart ever did. Mozart’s bad behavior was rooted in his youth and naïveté.
The quotation is part of Salieri’s first speech in act II:
“I was a good man, as the world calls good. What use was it to me? Goodness is nothing in the furnace of art.”
Salieri errs in asking this question—goodness, I would argue, is a virtue only if it does not seek to be rewarded. Doing an action that one does not believe in, for the purposes of gain, is a manifestation of the sin of pride. A true artist is not distracted by caring what the world calls them. The burning creativity in the “furnace” moves the artist to make genuine art, and social norms are a distraction to them. The metaphor of a furnace for creativity often appears as a “crucible,” where alchemists mixed and heated their transformative concoctions.