I have a fire door with a gap between about 3 and 4mm round the edges, but the cold smoke brush seal doesn't touch the door.  I'd say the gap is about 1mm.
Is that acceptable?

Thanks in advance.
 

The cold smoke seal should be in contact with the opposing surface in order for it to be effective.  There are various types of smoke seals available, including types that can seal against the face of the door instead of at the edge.