firebomb vs open fire

firebomb

verb
  • To attack with a firebomb. 

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

open fire

verb
  • To begin firing (weapons) at something or someone. 

noun
  • An uncovered fire. 

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