Question Author
Well, I've finally come up with the definitive answer, after much faffing around, from someone at Celtic View. Apparently there used to be football mag at the turn of the 20th century called Scottish Umpire (or Referee), and in this mag was a cartoon which featured fat cat club directors with bags of cash, who would refer to the Celtic/Rangers derby as a 'firm' because of all the money the fixture made. Interestingly, neither Rangers' nor Celtic's PR depts knew this.