Firm's demand curve is a curve showing relationship between price of the product (that the firm is producing) and its quantity demanded in the market. We know for a firm, price = AR. We also know that, firm's AR curve shows the relationship between price (or AR) and output (or quantity demanded) in the market. Accordingly, we can state that firm's demand curve is the same as AR curve of the firm. It is also called firm's price line .