Yes, you can use TFUEL to fund your account. Your TFUEL balance is maintained as a separate balance and you can use to buy NFTs, participate in auctions, Drops and more. TFUEL will use real-time exchange rate with market price at point of purchase. For example, when you buy a NFT, the exchange rate between TFUEL and USD at that point will be applied and used to finalize the transaction.
NOTE: If you pre-fund your NFT ahead of time, it's possible due to TFUEL price fluctuations at the point of purchase you don't have enough TFUEL. Please make sure you have enough TFUEL in your Balance before pre-funding for a Drop. Additionally, be sure to add your mobile phone number in Account Settings for notifications.