This reference occurs in section two, "The Sieve and the Sand." Montag has just been at Faber's house. Faber gives him the green bullet radio device to put in his ear so they can communicate at all times. War is on the horizon. Montag walks home. While eating his dinner, Millie, Mrs. Phelps, and Mrs. Bowles all come in and rush to the parlour room, presumably to catch the most recent show. But first, Millie runs from the parlour door to greet Mrs. Phelps and Mrs. Bowles "like a native fleeing an eruption of Vesuvius." Here, Bradbury writes that Millie runs from the parlour room as if it is a volcano, playing with the theme of fire and destruction. In this metaphor, the parlour room is destructive because it pacifies people into thoughtlessness. Then the women go back into the parlour (volcano) room:
Mrs. Phelps and Mrs. Bowles came through the front door and vanished into the volcano's mouth with martinis in their hands; Montag stopped eating.
As the metaphor goes, the volcano's mouth is the door to the parlour room. The room itself and its senseless (and destructive) shows are the volcano.
In Fahrenheit 451, the author has mentioned the volcano’s mouth in section two when Mildred’s friends come to Montag’s house. He says, “…Mrs. Phelps and Mrs. Bowles came through the front door and vanished into the volcano’s mouth with martinis in their hands…” The volcano’s mouth here refers to the parlor. The parlor consists of three large TVs on three of its walls and Mildred would like an additional fourth screen on the fourth wall. The red and orange colors emitted by TVs are similar to the fiery emissions of an actual volcano. In this case, the real disaster is the content aired through the TVs. People have been turned into zombies who stare at the walls throughout and know no world outside of the walls. In fact, Mildred, who spends all her days glued to the screens, is so entrenched in the artificial world displayed by the walls that she has a TV family. Therefore, volcano’s mouth represents the parlor.