1. Firm as: definition of firm from Oxford Dictionaries Online "a business concern, especially one involving a partnership of two or more people [as used in thefollowing expressions]:
- state support for small firms - a law firm"
Essentially I would generally use it in the case of a partnership providing "professsional services" such as a firm of lawyers, solicitors, accountants etc.
I would normally use this term in the context of the overall "parent company" that owns a significant group of companies or a large government-owned/controlled enterprise.