"'John Knox,' said Miss Brodie, 'was an embittered man. He could never be at ease with the gay French Queen."
John Knox House, Edinburgh
Public DomainJohn Knox House, Edinburgh - Credit: Masson, Rosaline Orme: "Edinburgh" (1912)
 John Knox (1510-1572) was a Scottish clergyman and a leader of the Protestant Reformation; he is generally considered the founder of Protestantism in Scotland.

He came into conflict with Mary of Guise, the second wife of King James V and mother of Mary, Queen of Scots, who acted as regent on behalf of her daughter between 1554 and 1560.

He also vehemently opposed Mary, Queen of Scots herself, even  though she was relatively tolerant of  the Protestant Kirk (the term used for the Protestant Church in Scotland).

Presumably, Jean Brodie refers to Mary as 'the gay French Queen' because of the French connections on her mother's side and her 18-month reign as Queen consort in France.

The house where John Knox is said to have lived is situated on the High Street, a section of the Royal Mile.