"The accused killed Laura Letts deliberately for financial gain," she said, noting he hoped to benefit from her family inheritance and insurance payouts.
"He would also collect her schoolteacher's pension for the rest of his days, which he would spend in the house she owned when they married."
Ms Firestone said Beckett took out a number of life insurance policies and accidental-death benefits on his wife between 2007 and 2010, the final one going into effect the month before Mrs Letts-Beckett died.
Ms Firestone said Beckett claimed to have no knowledge of the final insurance policy. But, she said, police found his fingerprints on the document.
Jurors were told Beckett was not immediately a suspect. He described the incident at times as an accident or suicide, Ms Firestone said.
"One by one, certain police officers and civilians began to realise that the version of events painted by Mr Beckett about the events of August 18, 2010, were not accurate," she said.
The victim's mother, Beth Letts, yesterday described to jurors how her daughter met Beckett while travelling in New Zealand in 1995.
Five years later, Beckett began making regular trips to Westlock, a town in the province of Alberta, where Letts-Beckett worked as a schoolteacher. The couple married in 2003. Mrs Letts said their relationship was a good one at first. She said Beckett eventually became "overbearing and domineering".
Mrs Letts-Beckett filed a police report in September 2007 alleging physical abuse at the hands of Beckett.
Mrs Letts said she was at the Westlock Royal Canadian Mounted Police detachment when her daughter made her statement to police.
"Laura was upset," she said. "She was crying. He [the officer] kept saying to her, 'Laura, it's not your fault'.
"He suggested to her that she leave the relationship. He said, 'I do not want to be dealing with a homicide later, Laura, and I strongly suggest that you do not go back'."
Mrs Letts said her daughter and Beckett separated for about three months in late-2007, reuniting in January 2008.
While they were separated, she said, Mrs Letts-Beckett came clean to her family about the fact Beckett had been married previously - something to which the Letts were staunchly opposed.
Mrs Letts said her relationship with her daughter after she reconciled with Beckett consisted only of birthday cards until a phone call on Mother's Day in 2010.
Less than four months later, Beckett called the Letts to say their daughter was dead.
Evidence continues.
The Crown expects to call 50 witnesses in the trial, which is expected to last three months.
