Howdy,
If you don't max perform the car in handling, there may not be a problem. If, you do push the limits on cornering and braking, then the front and rear tires need to be matched to provide equal performance.
The KUMHO CSTA LX PLATINUM tires are listed as being: "Kumho's Grand Touring All-Season tire developed to meet the needs of sporty coupe and luxury performance sedan drivers. It is designed to blend responsive handling, long wear and a quiet ride with year-round traction including in light snow."
The Firestone Firehawk comes is several different varieties, so you'll need to check them to see if they are also a "Grand Touring All-Season" type of tire.
If they are not matched in performance and capability, you may want to get rid of one or the other set.