FIRE is the intermediary token used in the Vulcan Forged marketplace. As bidding on items is quick fire, it is impractical to wait for every bidder to transfer their PYR to an escrow wallet before the bid is allowed (and then transfer it back again if they are outbid).
You convert your PYR to FIRE (1:1 ratio), and use FIRE to bid which is instant. You can then convert your FIRE back to PYR when you like.
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