When the fire occurs, the children see the town rally together to try to help one of their own. Even Mr. Avery nearly gets stuck in the window while the fire burns. The tension rises because the fire is a traumatic and violent event. Miss Maudie might have been killed. She loses much in the fire, but remains optimistic, noting how she'd always wanted a smaller house. Once again, Miss Maudie proves to be a good role model.
The tension, from Scout's point of view, is also augmented by Boo Radley's gesture of putting a blanket on her shoulders during the fire. She had no idea he had even been there. This adds to her sense of mystery surrounding the mythology of Boo Radley.
Scout adds that it is odd (and generous) that despite her tragedy, Miss Maudie continues to show interest in her life. Miss Maudie adds that she would have had the sense to turn around if Boo had been there. Miss Maudie is playing with Scout's infatuation but she's also subtly suggesting that Boo is not someone to be afraid of. So, the tension rises as the children observe how Miss Maudie and the town deal with the fire. The tension is also increased with Scout's continued misunderstanding of Boo Radley. Although, it seems that Jem has begun to understand Boo in a more sympathetic way.