burn at the stake vs firebomb

burn at the stake

verb
  • To kill (someone) by tying them to a post and setting a fire beneath them 

firebomb

verb
  • To attack with a firebomb. 

noun
  • A weapon that causes fire; an incendiary weapon. 

How often have the words burn at the stake and firebomb occurred in a corpus of books? (source: Google Ngram Viewer )