This is one of my favourite sayings. It is a great way to ensure you do your best to treat people the same way as you treat others. Firm but fair means that there is a firmness with which you are ok with because it is based on being fair. I believe it is easier to stand your ground on issues when you know you are being fair.
Sometimes the item to be done is a non negotiable. It really must be sorted out. This is where you need to apply some firmness. This may mean that you appear tougher on your staff or your children.
When you are being fair, it helps to know what is being discussed. Some understanding of how things have progressed to this point can be useful before weighing in on the situation. If one parent has said ok to something, demanding it be otherwise sends a confusing message.
One example of putting this point into practice is when the children are playing video games. It is often easier to extend their time and pretend that it is all fine.
Though the best thing to do is to be firm on the agreed playing time and assist them in transitioning to another activity. If you had agreed up front how things were going to be, you need to be firm in the face of challenge and opposition.
Firm but fair. Stay resolute.