It's all in the reference guide.
place one Fire token in a space adjacent to a space that contains Fire.
Two spaces are adjacent if they share a border, a door, an impassable border, or a wall.
Under Secret Passage:
Each space containing a Secret Passage is adjacent to each other space containing a Secret Passage.
So fire can spread to any adjacent space, with this exception listed under Wall:
Eﬀects that aﬀect adjacent spaces cannot aﬀect spaces through walls unless an eﬀect specifically allows it.
Since the fire effect doesn't specifically mention walls, fire is blocked by walls (but nothing else).