The bedroom is likened to a tomb and a mausoleum, dark, cold and airless. As he enters, Montag thinks:
It was like coming into the cold marbled room of a mausoleum after the moon had set. Complete darkness, not a hint of the silver world outside, the windows tightly shut, the chamber a tombworld where no sound from the great city could penetrate.
Despite a strong feeling of claustrophobia, he doesn't want to open the French windows, for he doesn't want moonlight to come into the room. He now associates the moonlight with Clarisse, who had just mentioned the moon to him.
The bedroom represents Mildred, who has sealed herself off from the outer world and its reality of moonlight and dew on the morning grass. He imagines Mildred on the bed, "like a body displayed on the lid of a tomb," her little seashells in her ears as she listens to her stories and music. He wants to keep her and the sterile world of death she represents--he will soon discover she has attempted suicide--compartmentalized, away from Clarisse and the life and clarity she symbolizes.
Montag's bedroom is repeatedly compared to a tomb. One of the elements that I had to remind myself of while visualizing the room is that it is described as completely dark, with no light at all, as though it were sealed shut. Montag isn't even able to see that his wife is nearly dead.
The room is described as having windows and could easily be a much more pleasant place if it were open to the moonlight, but Montag knows not to bother.
I think the room is largely meant to represent his wife, Mildred, who spends virtually all of her time with small radios in her ears, sealed off from the outside world, just like the room itself. However it also represents their marriage: cold, lifeless, and sealed shut, an ironic mirror image of what a marriage should be.
The bedroom is the one that Montag shares with his wife Mildred. It is a cold room, not inviting or homey. What is significant is that Montag refers to the room as empty, and then revises that to say not quite empty. This symbolizes Montag himself. He realizes in this scene, and in part because of his interaction with Clarisse, that he is unhappy. This realization has made him see that he is "not quite empty". There is still passion and desire in him, and they are the spark that will lead to his goals.