Can "Gift of Furious Might" be used on Rising Heat of Stone and Sand to increase the damage from "Gift of Searing Heat"?

Can "Gift of Furious Might" be used on Rising Heat of Stone and Sand to increase the damage from "Gift of Searing Heat"?

Yes. "Gift of Furious Might" lets a Spirit boost the Damage of one of its Powers by +3. "Gift of Searing Heat" is one of Rising Heat's Powers (even though it's working with/through another Spirit) so it qualifies.