A fire door is for emergency evacuation of the building. The route that forms the exit corridor to the outside must not be blocked at any time. If the fire door is an external door, it normally has a crash-bar on the inside to allow quick access from the inside. It cannot have a conventional lock on it.
A fire door is constructed of fire-resisting material and it prevents the spread of fire within a building. It normally has a self-closing device on it. A fire door may be a external door to the outside. However it can be lockable and may have a sign on it saying it should be kept shut at all times.
A fire exit door may also be designed to restrict the spread of flames, so may be made of fire-resisting material.
If your MIL allowed a fire exit to be blocked, the manager may be correct in his/her disciplinary action.
If this is merely a external fire door that was blocked, the manager doesn't understand the fire regulations as applied to business/commercial premises.
There's nothing to stop goods coming in or out through either a fire exit or a fire door - just that a fire exit shall not have stuff blocking the escape route.