A box of fireworks were set alight at a corner shop in Hounslow West on Friday evening.
Fire engines and police rushed to the shop in Basildene Road, just after 6pm, to find two shopkeepers still inside the room, which was damaged and full of smoke.
Firemen Simon Hall, of Feltham station, said: "They were remarkably calm considering what had happened. The box was still smouldering but was largely out by the time we arrived.
"They sell fireworks there, though, so I don't know exactly what happened. I think somebody had dumped them on the shop floor. It was lucky no one was more seriously hurt."
Firemen from Feltham station also attended a blaze the following day at The Centre, in Feltham. A fourth floor flat in Hibiscus House caught fire at 2.40pm but was put out before anyone was hurt.