But a few people were saying that this performance wasn't her best one : instead of using a backing track, she sang live and seemed to be having troubles with higher notes and was totally breathless by the end of the song.

My opinion is that people shouldn't blame her : this was one of her first times singing Firework live. It's quite a hard song to sing live and dancing in the same time. Personnaly, even though artists are sometimes just OK in their performance, I always love those that sing live. In this case, we can feel the energy, feel the words in a better way.

Anyway, I can easily understand that she was scared about how good Simon Cowell thought she was.