A firm is an individual enterprise or business unit under one control an ownership e.g. a business unit carrying the production of a good or service such as production of soap or a legal service firm.
A firm is a single business unit or enterprise under one ownership, management and control e.g. KCC, Brookside etc.
An industry consists of all those firms producing the same type of products in the same line of production. A sop industry consists of all those firms producing soap while an insurance industry consists of all these firms providing insurance services.
An industry refers to a group of firms producing the same products for a given market e.g. the milk industry which includes firms such KCC and Brookside. In some cases where we have a single firm, the firm becomes the industry.