The demand curve "as seen by the firm" varies with the structure of the relevant market. What is the structure for a firm that has to act as a price taker? Is such a firm doomed to suffer losses? How does the profit/loss situation change between the short run and the long run for such a firm? In the long run are average costs of production efficient?
What is the structure for a firm with at least some ability to determine price? How are price and output levels determined rationally? Since price can, at least to some degree, be determined by the seller, is this firm sure to enjoy profits? How might the long run differ from the short run?