Forge version: 1.12.2 - 14.23.5.2768
Twilight Forest version: 1.12.2 - 3.8.689

Steps to reproduce:

1. get a slab of any type, place one as the bottom half of the block
2. try to place a firefly below the slab
3. for a moment it shows the firefly as attached to the block below it, then drops as an entity.Also, firefly particles remain even after the firefly block pops off as an entity.

What I expected to happen:
firefly attaches to bottom half of slab, on the same level as the full blocks around it
What happened instead:
Wasn't able to attach, when you right click to place it, it shows the firefly as attached to the empty space below. I expected this to work because stuff like levers and buttons are placeable on the bottom surface of the bottom half of slabs, even if its a half slab and not a full block of 2 slabs. And after the firefly is unable to attach and drops as an entity, the particles of fireflies around it remain even after its gone. If the block is made a full block of 2 slabs on top of each other, it will stay though.