Officers are appealing for information on anyone seen carrying a box like the one pictured on the night of the fire in Tile Cross, Birmingham.
A murder investigation is under way after 56-year-old Anthony Nicholls died in hospital following the blaze at his home in Birchtrees Crescent last Thursday.
The fire started after the large commercial-sized firework was placed in the property and set alight at just after 11pm.
His partner, a 50-year-old woman, escaped with fractures to her lower limbs after jumping from a first floor window at the property.
Detective Inspector Paul Joyce, from Force CID, said: "We believe this was the type of firework used to start the blaze which killed Mr Nicholls.
"The box lid may have been discarded nearby and we are appealing for witnesses who saw anybody carrying it that evening or may know where it is now. This could help provide us with valuable clues.
"We owe it to the family of Mr Nicholls to find out who started the blaze and bring whoever was responsible to justice."
Information can be given to police on 101 or Crimestoppers anonymously on 0800 555 111.