Carmen Cairns, 28, set a mattress alight after falling out with her family last July.
Residents, including her mum Christine and a frail 93-year-old man, had to be evacuated as thick smoke belched through the building in Dennistoun.
The incident came seven years after Cairns's bid for stardom when Cowell called her and pal Karen Hunter "utterly useless".
The High Court in Edinburgh heard Cairns suffers from two personality disorders which prevented her from acting in a rational manner.
The court heard her family had forgiven her for starting the blaze.