Zuko, Exiled Prince

Mana from firebending abilities isn't lost until you leave combat and go to your second main phase. It can be used at any time during combat, even after combat damage has been dealt.

You must follow all normal timing rules for a card you play using Zuko, Exiled Prince's last ability and, if it's a spell, you must pay its costs to cast it.

"Firebending N" is a keyword that represents the ability "Whenever this creature attacks, add N {R}. Until end of combat, you don't lose this mana as steps end."