Firebugs are known for their gregarious behavior; they like to be together. To attract each other, they use aggregation pheromones. These gatherings can be observed on structures, plants, and even under leaf litter.
An onlooker finding such a large group of firebugs might become alarmed, but it is nothing to be worried about. Sometimes these groups may have only a few bugs, but at other times you may see hundreds of them. This behavior is believed to keep them warm up and protect them against predators.
I watched a congregation of firebugs in our garden yesterday. What a bustling event this was.