Third party fire and theft is third party property damage with two additions: it also covers your own car if it is stolen, or if it catches fire.
It is the middle rung. You are covered for the two losses that wipe out the whole car without anyone hitting it, but not for collision damage to your own vehicle.
Theft cover pays out if the car is stolen and not recovered, and covers damage if it is recovered in worse condition. Fire cover responds to the car burning, whether from an electrical fault, a nearby fire or arson. Both are typically capped at a set sum insured that is lower than a comprehensive policy's agreed value, so read the limit rather than assuming it matches the car's worth.
Collision damage to your own car is still not covered. If you run into something, or something runs into you and the other driver is not identified, you carry that cost yourself. Hail, storm and flood are also generally outside this level. For a lot of drivers those are more likely than theft, which is worth weighing before choosing this over comprehensive.
It fits a car that is worth enough that losing it entirely would hurt, but not enough to justify comprehensive premiums — and particularly a car parked on the street or in an area where theft is the realistic risk. If the car lives in a garage and the main risk is a carpark scrape, the fire and theft component may be paying for a risk you do not have.
Comprehensive covers accidental damage to your own car as well. Fire and theft only covers your own car when it is stolen or burns; if you hit something, the repair is yours to fund.
Generally no. Weather damage sits with comprehensive. Given hail can write off a car in minutes, that is worth factoring in if you are in a hail-prone area.
It depends on where the car is kept and what it is worth. If it is parked on the street in an area with real theft risk and replacing it would hurt, the step up is usually small. If it is garaged, you may be buying cover for a risk you do not carry.
Medicare Levy Surcharge thresholds and government rebate percentages are reviewed regularly, so this page does not quote them. For the current figures see the ATO and the government's privatehealth.gov.au.