So did many other countries. The US is getting those doses faster by preventing the Pfizer factories in the US from exporting any doses, so they have no choice but to sell all of that supply to the US. That's why Canada is getting all of the Pfizer/BioNTech doses it purchased from the Belgium factory rather than one right across the border in the US.
Far from firm? If I sign a contract to buy apples from you 6 months from now, but you show up with apple seeds, was my order not firm if I don't pay you?
The fact is when Pfizer showed up with a vaccine, they knew they could sell $2 billion to the US and would have legal recourse if the US government didn't pay. That's as firm as it gets.
The participants in operation warp speed get paid whether or not their vaccine makes it through trials.
To use your analogy, farmers can sign contracts for "all the apples that's produced this year" if they want to offload the risk of crop failure rather than a contract for a specific number of apples.
A `firm order` is a business term to mean an order which is non-cancelable. It would be correct to say Pfizer was only tangentially connected to Warp Speed as Warp Speed money was used to place the firm order of $2 billion doses with Pfizer.
Pfizer knew for a fact, even if COVID was miraculously cured, or it turned out there was a super cheap treatment, or anything that made their vaccine worthless, they could still sell $2 billion units to the US.