Kirshner was born in Toronto, Ontario, the daughter of Etti, a teacher, and Sheldon Kirshner, a journalist who wrote for The Canadian Jewish News. Kirshner is a granddaughter of Holocaust survivors; her father was born in a displaced persons camp in Germany in 1946, and met Kirshner's mother, a Bulgarian Jewish refugee, after they escaped to Israel. Kirshner's paternal grandparents were Jews from Poland. Kirshner had a middle class upbringing and attended Forest Hill Collegiate Institute but later graduated from the Jarvis Collegiate Institute. Kirshner studied Russian literature and 20th-century movie industry at McGill University in Montreal. Her younger sister, Lauren Kirshner, a writer, was involved in the I Live Here project.