回複：可惡的房東，要求我7日內支付那筆巨大的賬單

Your landlord's insurance covers his/her loss, but does not cover your liability. You are liable for your fault, assuming the fire was your fault.
Unless it is specified in the rental contract or if you have agreed, your landlord cannot go ahead to have the damage fixed and bill you the cost. You may dispute that the cost is unreasonably high.
Bottom line, you must pay the reasonable cost to fix the fire damage.
You should have purchased renter's insurance.