Netwon's first law refers to object under the influence of zero forces. We have chosen to expand on that to consider systems with zero net force. Here are some examples of systems that have forces acting on the system, but the net force for the entire system is zero and thus in Equlibrium. 

A box with wheels attached to bottom is on top of a horizontal scale. Attached to one side is a horizontal rope with a spring scale to measure force (assume the scale and ropes are mass-less). On the other side of the box is a rope that has no force being applied to it.

Sketch a FBD of the box

A 10-kg-box with wheels attached to bottom is on top of a horizontal scale. Attached to one side is a horizontal rope with a spring scale to measure force (assume the scale and ropes are mass-less). On the other side of the box is a rope that now has a horizontal force equal to 150 N pulling on it.

Sketch a FBD of the box, attempting to scale each vector relative to each other.