Brimstone most likely refers to sulfur. It is a bright yellow, highly combustible mineral that puts out a terrible odor when it burns. Brimstone is abundant in the area around the Dead Sea. During earthquakes hot gases are released from inside the earth, which can cause the brimstone deposits to burst into flame (29:23).
Shoes and sandals were considered the most humble articles of clothing. They were removed at the doorway before entering a home. People also removed their shoes during periods of mourning. Going barefoot was a sign of poverty and shame (25:9–10).
What was an upper millstone? Israelite women ground their grain in mills made of two circular stones. The grain was placed on the flat lower stone and then crushed as the upper millstone rolled over it. To deprive the miller of either stone would make her work impossible (24:6).