In my experience, being firm in the classroom requires a balancing act. You need to let students know what you expect, and be fair in your expectations, but I believe you also need to be human. "Firm" denotes "solid," but in education, I think it must also mean "safe."
When students know what you expect, that you are fair and that you care for each one of them, they will learn that "firm" allows you to teach the class, but also to be supportive of them as they need it; often there are kids in your classroom that have no support and no structure at home to make them feel safe in their world. "Firm" does both: it's like "solid with a heart."