I've been running the Mount Hyjal storyline so I can get into the Molten Front (for a special recolored transmog belt that you can't get anywhere else), and it is mentioned several times that flying by normal means in the Firelands is impossible thanks to "dense, superheated air."
Now, I know a decent amount about science and physics, but... I do not understand how this would make flying impossible? Is it like water, where if you go deep enough, the density of the water becomes so extreme that the human body will actually begin to sink? (Except this is applying to air, which is just another a fluid like water mechanically speaking), with the heat increasing pressure (How exactly does heat make the air dense)?