What can I do if an insurer is filing a claim against me for a fire but there is no fire report blaming me as the cause of the fire?

A fire report is not a legal conclusion or court adjudication: it is not binding, and it is not necessary for someone (e.g. the insurer) to file a claim or lawsuit. (It's certainly helpful, as it provides evidence, but it is not required.) You don't have to pay unless and until they sue you in court and win; if they do sue, you can defend in court by providing testimony and evidence that you did not cause the fire and/or that someone else did--and remember: as the party suing you, they have to prove their case, which means that unless they come forward with credible evidence that you caused the fire, you would win. If you do lose and can't or don't pay, they can try to take collections actions against you, such as putting a lien on real estate, levying on bank accounts, garnishing wages, or executing on (seizing and selling) valuable personal property, like cars. If you believe they could win in court (show that you did cause the fire), you could also try to work out or negotiate some settlement with them that you could afford to pay, though it may be that anything you could afford to pay is not enough for them and they might reject the offer (settlements are voluntary). Generally, the other side will accept less in order to avoid the cost of a lawsuit, so you will  probably be able to save something with a settlement (though the questions are, how much, and will it be enough?).