The MQ gas sensors are not very accurate and they detect a number of gasses (you don't know which one it is detecting).
This is a start page for the MQ sensors: Arduino Playground - MQGasSensors
The best way to describe the sensors is : "The impedance of the sensor is lowered, if something is detected".
For the air quality it is possible to use them. When you look at the sensor value for a few weeks, you should be able to tell if the air quality is getting worse.
On the other hand, the sensors are very sensitive.
The MQ-2, MQ-7, MQ135 will change a lot when something is smoking.
You could use all three to start with.
A flame detector could be an infra-red flame detector. If you want something like that, you have to buy a reliable (and expensive) flame detector.
Properly placed smoke detectors are reliable. But they are ment to warn to get out of the house, not to prevent a fire.