As the climax of the novel, "Burning Bright" contains many varied tones, from the anger and sadness Montag feels when he destroys his house and kills Beatty, to the fear of being chased through the city by the unstoppable Mechanical Hound. The following lines call back to the opening of the novel:
And as before, it was good to burn... Fire was best for everything!
Later, as Montag escapes the city and meets up with Granger, he begins to feel a sense of relief; his structured, organized life is over, and a new life has begun, one in which he can make his own decisions and follow his own path:
The river was very real; it held him comfortably and gave him the time at last, the leisure, to consider this month, this year, and a lifetime of years.
Finally, Montag feels unburdened, and the tone of the book -- which shifted back into tension when the city is bombed -- echoes his relaxation:
They finished eating and put out the fire. The day was brightening all about them as if a pink lamp had been given more wick. In the trees, the birds that had flown away now came back and settled down.
(Bradbury, Fahrenheit 451, Google Books)
It might be possible to classify all of "Burning Bright" as cathartic in tone, because it comes after everything else and serves to tie up loose ends and leave the characters in a new place; however, such varying tones do not lend themselves to a single interpretation. The main tones would be anger: Montag burning the house; fear: Montag fleeing the city; and satisfaction: Montag realizing that he has a purpose now, something beyond destruction, and something to strive for in the future.