At this moment, the inner part of the Fiery Nifty Parts don't exactly connect to each other. This leaves gaps in between the block model. Having it connect to each other will make it act more akin to the Block of Fiery Metal. This can only apply to blocks that are similar, so the Fiery Blocks don't connect to each other, and the Double Fiery Slab does not connect to a Block of Fiery Metal.

As Twilight Forest compat has been moved into a separate mod, this is now an issue for Do I am Ornament.