Fires of Invention

• WWizards of the Coast2019-10-04
  Official Ruling
  You may choose to pay the mana cost or an alternative cost for spells you cast, even if their mana value is less than or equal to the number of lands you control.
• WWizards of the Coast2019-10-04
  Official Ruling
  If a spell has in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.
• WWizards of the Coast2019-10-04
  Official Ruling
  If you cast one card twice during a turn (such as by casting it as an Adventure and then casting it from exile as a creature), you've cast two spells.
• WWizards of the Coast2019-10-04
  Official Ruling
  If you cast a spell that was countered, it still counts as one of your two spells.
• WWizards of the Coast2019-10-04
  Official Ruling
  Fires of Invention will look at the entire turn to see if you've cast two spells yet that turn, even if Fires of Invention wasn't on the battlefield when those spells were cast. Notably, you can't cast Fires of Invention and then cast two more spells that turn; if Fires of Invention was your second (or third, or fourth) spell that turn, you can't cast another spell at all.
• WWizards of the Coast2019-10-04
  Official Ruling
  If an effect says that you may cast a card that you couldn't otherwise cast if you pay an alternative cost (such as the flashback keyword), you have to pay that alternative cost if you use that permission to cast the card. You can't choose to cast it but cast it without paying its mana cost.
• WWizards of the Coast2019-10-04
  Official Ruling
  If you cast a card “without paying its mana cost,” you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs (such as that of Fling), you must pay those to cast the card.
• WWizards of the Coast2019-10-04
  Official Ruling
  Fires of Invention doesn't impose any restrictions on playing lands or activating abilities.
• WWizards of the Coast2019-10-04
  Official Ruling
  If an effect allows or instructs you to cast a spell during another player's turn, you can't do so. The restriction of Fires of Invention takes precedence over that permission. Similarly, if an effect allows or instructs you to cast a third spell during your turn, you can't do so.

Rulings via Scryfall

• WO
  Wizards of the Coast2020-07-13

  Fires of Invention was banned in historic on July 13, 2020. Because we feel the data shows a positive impact from the current suspensions, and because it is unlikely to be safe to reintroduce these cards to Historic anytime soon, Fires of Invention is banned in Historic. Source: https://magic.wizards.com/en/news/news/july-13-2020-banned-and-restricted-announcement-2020-07-13
• WO
  Wizards of the Coast2020-06-01

  Fires of Invention was banned in historic on June 1, 2020. For similar reasons as in Standard, we're choosing to suspend Fires of Invention and Agent of Treachery in Historic. As we continue to watch Historic grow and monitor the evolution of the metagame, we will in the future reevaluate whether it's appropriate to unsuspend each of these cards or whether these suspensions will become bans. Source: https://magic.wizards.com/en/news/news/june-1-2020-banned-and-restricted-announcement
• WO
  Wizards of the Coast2020-06-01

  Fires of Invention was banned in standard on June 1, 2020. Over the course of the last several weeks, Fires of Invention decks have risen to have a dominant win rate and metagame presence in Standard, achieving a 55% win rate and having even or favorable matchups against each of the other top ten archetypes. This indicates that metagame forces alone aren't sufficient to keep the deck in check. Because of the Fires of Invention deck's current high win rate and metagame share, and the risks and design constraints it poses to the environment going forward, Fires of Invention is banned in Standard. Source: https://magic.wizards.com/en/news/news/june-1-2020-banned-and-restricted-announcement