How does Fahrenheit 451 end with a feeling of hope and promise?
Most of Fahrenheit 451 focuses on the vices and negative tendencies of human nature that mades Montag's society so dark and miserable. So it is nice that at the end of the novel, there is a ray of hope. If you look at only the events of the ending, it still might seem quite cynical and depressiong; Montag's city has been decimated, his wife killed, Faber has suffered an unknown fate, Clarisse is gone, and everything that Montag ever knew or loved was destroyed. That's not a very happy picture.
However, if you look closely at the plan that Granger outlines, and the story of the phoenix that he uses, there is a ray of hope and optimism. Granger describes how according to legend, the phoenix, a bird,
"every few hundred years he built a pyre and burnt himself up...but every time he burnt himself up he sprang out of hte ashes, he got himself born all over again."
This story relates to Montag's society; it has crumbled and burned, and these men, these outliers, are going to be the ones to go back and rebuild everything. Granger says that as they rebuild, this time, they are going to rebuild things right so that they can stop making all of the same mistakes.
So despite the dramatically depressing ending, Bradbury takes the ashes of Montag's society, and asserts that a brighter, better world can be built out of them, and Montag will be one of the people who helps to do that. That is the feeling and hope that lies at the end of the novel. I hope that those thoughts helped; good luck!