I envision the "parlor walls" as being similar to massive flat-screen TVs that run floor to ceiling and span multiple walls in a room. Montag and Mildred have three walls in one room that are now dedicated to these large screens, and Mildred wants a fourth to complete the room. However, the cost is so exorbitant that Montag tries to dissuade his wife from pushing the issue.
Mildred is so addicted to the programming on these walls that she considers the people who appear in the programming as her "family." In fact, when Montag asks her to turn off the screens for a while, she replies, "That's my family," feeling bound and connected to a false world. The parlor walls have replaced meaningful human interactions, and Mildred's suicidal tendencies reflect the innate harm in this loss of genuine human connection.
Bradbury's vision was quite impressive considering his setting. When writing this novel in the early 1950s, televisions were large box-like structures that took up a great deal of space. Programming was in largely black and white, and in 1950, only around twenty percent of homes owned a television. Yet the power of television was evident, and within a decade, around ninety percent of homes owned a television. Families gathered around a television to share programming together, and shows such as Fireside Theatre, I Love Lucy, and The Price Is Right became popular forms of entertainment.
While these large screens spanning the length and width of entire walls may not seem all that far-fetched to our modern and screen-dependent world, it was certainly far beyond the technologies which existed when Bradbury penned his novel. Bradbury nonetheless saw the power television exerted over the populace and realized the dangers inherent in spending increasing amounts of time in front of screens, a message that has become increasingly appropriate for modern societies.