Lyric discussion by banananana 

Cover art for Mirror lyrics by Ellie Goulding

I haven't found it written out on any lyric sites yet, but I could swear that in the last chorus, it goes:

"I was the girl who was on fire (A hero) I was the girl who was on fire (A hero)..."

and then the rest like it's written. It may be just a background vocal effect, but I feel that it's really powerful and extremely important to the song.