Robert Price, 76, was pronounced dead shortly before 6pm on July 27 at his home in Oval Road North, Dagenham.
Nathan Otitodilchukwu, 18, previously of Crow Lane in Romford, pleaded guilty to manslaughter.
A 16-year-old, who cannot be identified due to his age, has also admitted manslaughter.
Otitodilchukwu is not said to have thrown the firework himself but is said to have been part of the group who started the fire, the Old Bailey was told at a previous hearing.
The pair were due to be sentenced on Friday, April 25, but the hearing was pushed back to May.
East London Coroner’s Court previously heard that “a group of males were seen to approach the deceased’s home address and throw a lighted flare into the property”.
A fire broke out, senior coroner Graeme Irvine said, and Mr Price’s death was declared by paramedics from the London Ambulance Service.
A forensic post-mortem on July 29 has since given Mr Price’s cause of death as “inhalation of fire fumes and burns”.
