question
The fire finishes its triumphant run down Main Street by engulfing Food City,
then rolls onward toward Dipper’s, where those still in the parking lot scream and clutch at each other. Their last sight on earth is of a firewall a hundred yards high running eagerly to meet them, like Albion to his beloved. Now the flames are rolling down the main roads, boiling their tar into soup. At the same time it is spreading into Eastchester, snacking on both yuppie homes and the few yuppies cowering inside. Michela Burpee will soon run for her cellar, but too late; her kitchen will explode around her and her last sight on earth will be her Amana refrigerator, melting.

Question 1: Why does the author add an "of" after "is", but not add an "of" after "will be"?

Question 2: Can I take them as the same meaning no matter whether adding "of" after “is”or not?

Question

question
The fire finishes its triumphant run down Main Street by engulfing Food City,
then rolls onward toward Dipper’s, where those still in the parking lot scream and clutch at each other. Their last sight on earth is of a firewall a hundred yards high running eagerly to meet them, like Albion to his beloved. Now the flames are rolling down the main roads, boiling their tar into soup. At the same time it is spreading into Eastchester, snacking on both yuppie homes and the few yuppies cowering inside. Michela Burpee will soon run for her cellar, but too late; her kitchen will explode around her and her last sight on earth will be her Amana refrigerator, melting.

Question 1: Why does the author add an "of" after "is", but not add an "of" after "will be"?

Question 2: Can I take them as the same meaning no matter whether adding "of" after “is”or not?

Answer

Using "of" is optional! In the sentence "their last sight on earth is of a firewall a hundred yards high," the of is implying that "their last sight on earth is THE SIGHT of" and so on. You could also say "their last sight on earth is THAT OF a firewall," where that means "the sight." All three options mean the same thing.
However, I personally think using "of" like that is kind of clunky, so I'd avoid it unless you want to go for a dramatic effect.