Peter Lomas, 15, started the fire at the Littlewoods store in Chesterfield, Derbyshire, because he 'liked seeing fire engines in action'.
Two pensioners, died after being trapped in the store's cafeteria as the fire spread through the building in May this year.
Mr Justice Drake told Lomas, of Old Whittington, Chesterfield, that he would be locked up until he was no longer a danger to the public.
Nottingham Crown Court was told Lomas had watched the rescue operation and within days had set three more fires in shops and the town library. 'He said he chose big shops because they would attract more fire-engines,' Andrew Hamilton, for the prosecution, said.
In September this year Lomas also set fire to his room at a secure unit in an attempt to kill himself.
Lomas admitted two charges of manslaughter and three charges of damaging property being reckless as to whether life was endangered, all in May this year. He also admitted a further charge of reckless arson in September this year at the secure unit where he was being held.Reuse content