You have just done your own argument in by describing the term as to 'describe a rivalry between two football clubs and their fans'. The 'Old Firm' rivalry was between Celtic FC 1888 and Rangers FC 1872. The Club currently playing at Ibrox is not the Club formed in 1872 so how on earth can the 'Old Firm' term be used as anything other than in the past tense? Would people reporting sightings of Elvios Pressley on the Las Vegas strip 'In Common Speech' be regarded as FACT that he was alive. I rest my case. This example of page hijacking by fans of Rangers FC 1872 has made Wikipedia nothing other than an embarrassment as a source of information.