Does it have to survive multiple fires with no maintainance, or just survive the fire, then can be cleaned / serviced
How close to the fire front do you need it to monitor, and do you need to monitor any of the data types while sitting in the fire
The main issue I see is temperature limits, mostly for the gas sensors and humidity sensors for anything above 200C, and the copper wire used to connect it for much over 800C (almost no strength) anywhere above ground will see them melted. temperature is easy so long as your not above about 1600C (a quick google seems to hint about 800C, so with some good margin)
If I had to actually build one, I would have the main electronics underground, probably about a half meter, with the gas and humidity sensor riding in a blooming flower type mechanism so it ends up down the hole underground once the fire gets too high, leaving just the thermometer, which you can buy wither PTC1000 or thermocouple type sensors rated to deal with and have that wired back to your electronics
Once the fire front has passed, you clean off any debree so the sensor can bloom again and start monitoring, or remove it until the next fire risk time, if left in the field you going to have to deal with plants and insects jamming mechanisms
If you really want to monitor gas and humidity during the fire, there is a work around, but it comes at the cost of electricity on your part, you need to siphon down a small flow rate of air, cool it down to the working range of the sensor, with either water for short duration fires, or a peltier for longer duration fires. with the gas cooled, you can safely measure the coposition during the fire without risking melting the electronics,