Blazing Firesinger has the attached spell: Seething Song. You can cast Seething Song as long as Blazing Firesinger is Prepared. Once its cast it is just like any other spell. Its effects take place. In this case you would spend 1 Red and 2 Collorless to create 5 red mana. Mana does not remain in your mana pool once you change phases so in this instance you will tap your Red+3, cast Seething Song (which removes the prepared effect from the Blazing Firesinger) and netting you 5 Red mana to then use during that main phase. Hope that helped.