Hi Veronica,maybe the significance of "fire" and "enthusiasm" sounds quite similar,but there's still some differences between them in this case:
1,"Fire"(noun) means "fervour,spirit,vivacity" and it's often used like literature like "poetic inspiration".
2,"Enthusiasm"(noun) means "strong interest or admiration,great eagerness,or keenness...about/for st"---and in our daily life,we often use "enthusiasm" more than "fire".
Do you understand?Feel free when using them,which words to use is to depends on context or expressive nuance.