Fireborn is a gimmick.
With a full super, Fireborn Sunsingers should get a buff to damage, similar to Weapons of Light.
That single change might fix this broken subclass.
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... Except that Fireborn is actually so clutch I got sick of constantly needing to run it in Year 1? It's value is more situational in PvP, perhaps, but it's better than a Gjallarhorn in PvE sometimes.