I believe that certain accessories such as the Ankh shield and Terraspark boots should provide an immunity to the On Fire debuff. It doesn't really make sense that you can be immune to fire blocks and lava, yet still be set on fire in the late game (especially when wearing solar armor). It's admittedly a minor annoyance, but one that I feel compelled to suggest nonetheless. It's a similar feeling to being slowed by cold water prior to 1.3 while wearing the Ankh shield. Just feels weird to not be immune to such a thematically minor debuff IMO.