Sozin's Comet
Sorcery
Oracle Text
Each creature you control gains firebending 5 until end of turn. (Whenever it attacks, add
. This mana lasts until end of combat.)
Foretell 
(During your turn, you may pay and exile this card from your hand face down. Cast it on a later turn for its foretell cost.)
Rules for this card
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A keyword ability that lets a player exile cards from their hand and cast them for an alternative cost on future turns. See rule 702.143, “Foretell.”
Rulings (8)
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Wizards of the CoastIf you're casting a foretold card from exile for its foretell cost, you can't choose to cast it for any other alternative costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, those must be paid to cast the spell.
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Wizards of the CoastCasting a foretold card from exile follows the timing rules for that card. If you foretell an instant card, you can cast it as soon as the next player's turn. In most cases, if you foretell a card that isn't an instant (or doesn't have flash), you'll have to wait until your next turn to cast it.
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Wizards of the CoastBecause exiling a card with foretell from your hand is a special action, you can do so any time you have priority during your turn, including in response to spells and abilities. Once you announce you're taking the action, no other player can respond by trying to remove the card from your hand.
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Wizards of the CoastMana 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.
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Wizards of the Coast"Firebending N" is a keyword that represents the ability "Whenever this creature attacks, add N. Until end of combat, you don't lose this mana as steps end."
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Wizards of the CoastMultiple instances of firebending on the same creature trigger separately, each granting you the appropriate amount of mana.
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Wizards of the CoastAny other mana you add during combat will still be lost as normal when moving between steps of combat.
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Wizards of the CoastFirebending abilities aren't mana abilities. They use the stack and can be responded to.