I am concerned about the light that appears under my kiln lid during firing.

The light that you see under the lid is normal, as long as the lid is not rising in front. The inner lid surface expands more than the outer cooler surface. Therefore, it bows inward slightly toward the firing chamber. This is normal. For this reason, you will see a thin line of light around the lid, since the lid is no longer a flat surface resting on the flat firing chamber sidewalls. Surprisingly, there is very little heat loss from around the edge of the lid.