First Firing of your New Kiln
I have just been reviewing information on kiln elements. I have discovered the reason you need to do your first firing with the kiln empty of everything. No kiln wash, no kiln furniture, nothing. Vacuum the kiln to take out any dusts from travel.
The element forms a protective layer of aluminum oxide during the first firing. If there are elements of kiln wash, dusts, or glass, this will inhibit the ability of the oxide coating to be uniform. The uniform coating of the elements is important to the long life of your elements. There are other things of course, but this is the initial, essential element of preparing you kiln for use.
After this first firing you can add the other elements of kiln wash, furniture, and even glass.
In summary, fire your kiln clean and bare. No kiln wash, no furniture.