This happened in Season 5, Episode 1.
In a nutshell: Edith is sad about her missing lover - Michael Gregson and the fact that she cannot be with their child - Marigold.
That night, out of anger, frustration, and basically all the feels, she throws a book that Michael had once given her across the room and falls back into bed.
The problem is, the book hits the fireplace and catches on fire.
![](https://s2.save4k.ru/pic/ZrJvoaooICA/mqdefault.jpg)