|The following information is from the Adventure Time comic book series.|
It may not be part of the same canon or continuity as the animated series.
Adventure Time with Fionna and Cake Issue 5 is the fifth issue of the six part miniseries by BOOM! Studios starring the gender-swapped characters.
Fionna is returning with her wand and meets Marshall Lee, who is going home. Fionna goes downstairs and discovers Prince Gumball's oven was able to temporarily restore Flame Prince to his normal form and that Cake gave him new clothes. When Flame Prince tries to touch Fionna, he sets her hat on fire and when Cake puts it out, it hurts him. Gumball explains that he raised his body temperature, so he's too hot to touch but at midnight he'll revert to his weakened state as long the heated salt crystal is inside him.
Prince Gumball reveals that he informed Ice Queen that the fire lion cubs she captured belong to a prince. She agreed to return them if Flame Prince goes on a fancy dinner date with her. They disguise Fionna and Cake as a Fire Lion and they and Flame Prince go to the Ice Kingdom to try and trick Ice Queen into getting the heated salt crystal out of his heart.
When they arrive at the Ice Kingdom, Ice Queen invites them in and reveals the frozen fire lion cubs and freezes Flame Prince, seeing easily through Fionna and Cake's disguise. Fionna attacks her but is easily subdued by Ice Queen who holds her by her hair and prepares to freezes her until Flame Prince shoots fire at her hand and accidentally sets Fionna's hair ablaze.
Back up story
Cootie Power Part 2
Fionna and Cake see a herd of stampeding "sinorses" (horses with noses for heads) who are heading towards Prince Gumball's castle. They get ahead of them and Fionna orders them to stop. However, Fionna still hasn't bathed and the sinorses run away in repulsion. Grateful, Prince Gumball prepares to make a law requiring no one to bathe more than Fionna for safety, until he smells Fionna again. It then rains, giving Fionna the bath she needed and she is finally allowed into Prince Gumball's lab and the story ends, revealing Fionna still has cooties.
Cootie Power Part 2
Comic pages and panels