In one sense, you are right, as the novel ends after a major event that has clearly ended civilisation as it is known in this book. The bombing of the city that Montag witnesses will obviously have massive repercussions, and we leave Montag and Granger and the other book people trekking towards the ashes of the city to see who has survived.
However, on the other hand, we could similarly argue that this is not actually a cliffhanger, as a number of the key elements of the plot have been resolved. Montag now is completely safe as those who were pursuing him are all dead. He is now able to live in a way that enables him to keep on reading books and to be part of shaping the future. The central conflict of the novel, in this sense, is over, and the way the text ends seems to point towards a brighter and happier future. Note what the quote from the Book of Revelations that Montag recites says:
And on either side of the river was there a tree of life, which bare twelve manner of fruits, and yielded her fruit every month; And the leaves of the tree were for the healing of the nations.
Such a reference to the "tree of life" and the "healing of the nations" clearly suggest a chance of renewal and a bright hope for the human species, which will rise out of its own ashes like the phoenix Granger has just told Montag about. Although the text ends with them thinking about what will happen when they reach the city, the major conflicts have been resolved, and therefore I would argue that it does not end in a cliffhanger.