No, humans are not immune to fire - but when molotovs were properly deployed they could save them - just throw one BEHIND zombie group - noise from crashing bottle and fire should get their attention and they should walk into fire. Even loud cellphone and alarm clock would be more useful than guns... they could even improvise molotov right on spot - get glass bottle(i bet they have one of them) fill it with fuel from car - and let it burn.