What is the description of the sitting posture of one who is firmly seated, and is a person sleeping in a vehicle or on a chair, with their back supported, considered to be sitting in the posture of one who is firmly seated or not?

The posture of a 'firmly seated person' (al-mutamakkin) by which ablution is not invalidated, according to some scholars, is: that the sleeper has their posterior firmly placed on the ground or whatever they are sitting on. This applies whether it is on a car seat or anything else. This is considered a firm posture even if their back is leaning against a wall or something similar. This is the madhhab of Imam Al-Shafi'i.