In a recording played in Brisbane Supreme Court, Mason told a man he thought was a fellow inmate that he started the fire after an argument with Alexis Parkes.
When the man, who was actually an undercover police operative, asked what was he thinking at the time, Mason said: "I will get you back bitch, you don't f***ing threaten my family."
Mason later laughed and told the man: "You know what (someone) ... said when I told her I done it (fire) - did you get marshmallows?"
Ms Parkes, 50, died days after emergency services found her unresponsive in her burning house in Brisbane's south in February 2020.
Mason has pleaded guilty to arson at his trial.
However, the 46-year-old has entered a not guilty plea to murder after telling police he did not consider the fire's consequences and had no intention of hurting his partner of 11 months.
Mason spoke with the undercover operative in the Brisbane Watch House the day he was arrested, while Ms Parkes was still fighting for her life in hospital.
Earlier that day, he told police in interviews played in court on Wednesday that he had set alight a car parked underneath Ms Parkes' Chermside home after a fight.
Mason told police Ms Parkes had gone "psycho" on medication and kicked him out about 7.30pm after threatening him and his family.
He told officers he went downstairs to sleep but she kept "carrying on", stomping on the floorboards above his head and yelling more threats.
By 3.30am Mason said he put a few capfuls of petrol on and near the car - situated underneath Ms Parkes' bedroom - before igniting it with a candle and leaving.
"I should have gone back and put it out ... I just didn't think about it (consequences)," he told police.
"I had no intention of hurting her ... just to pay her back for threatening my family."
Asked if he had ever hurt Ms Parkes physically, Mason said: "No, she had been violent with me and I just walked off. I am more of a man to walk off then lay a hand on a woman."
In another interview later that day, he told police he heard Ms Parkes walking around inside and the front door opening and closing before he left the house fire.
Four days after she was found by firefighters, Ms Parkes died of multiple organ failure due to injuries from the blaze.
That same day police told Mason of his partner's death and interviewed him again.
This time Mason told them he tried to raise the alarm after setting the car alight.
"I came to realise 'what the eff am I doing?' I come out screaming underneath the house 'honey, hey come on, get out quick, the house is on fire'," he told police.
"I heard nothing."
Mason said he then headed to a bus stop "walking up the road in tears".
The trial before Justice David Jackson continues.
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