1/2" drywall over firewall in kitchen?
I doubt if it’s a fire wall that would require 5/8” gypsum board. The only place I can think that 5/8” Type-X (fire rated) gyp board is required is on the garage side of a common wall with an attached garage or  on the ceiling of 1-hour fire separation between units, or on a wall next to a furnace room or garbage room. 

If any of the old gyp board is still around, you could look on the back and see if there are any fire ratings stamped on it. You could also look on the other side of the wall to see if a “Fire Rated” board is used on the other side of the wall. 

There are “Fire Partitions “ required between units (or sleeping rooms), but that is for a building with 3 or more apartments and you could use 1/2” fire rated gyp board. 

Remember, not all 1/2” or 5/8” gyp board is fire rated. If it’s a true fire wall (or fire partition) it must have a fire rated stamp on it and it needs to be applied on both sides of the wall, except between the garage and apartment. 

There is a sound control rating required between units, but that does not require a fire rated board.