Years before a certain prophesy would come to fruition, a young pegasus filly fell from the sky near Las Pegasus, only to be rescued and taken in by Burning Sky and Dragonfly, a newlywed pegasi couple on leave from the Equestrian Air Guard for their honeymoon. Adopted and named Firefly by the very ponies who caught her out of the sky, the young filly is secretly a being from another reality, a former human who had been a fan of a popular show that had portrayed this very world (or one very similar to it), with no idea what had brought her here, why she was now a pony, or even when in the timeline she happens to be.
Once she figures out that she's in the past, with years before Princess Luna returned as Nightmare Moon, young Firefly resolves to simply stay out of the way and let events happen as they should. However, once she sees Princess Twilight Sparkle and Spike the Brave and Glorious appear out of a bubble of complex looking magic, Firefly feels an incredible sense of liberation, believing that it no longer matters what she does or doesn't do as everything would correct itself once Twilight dealt with Starlight Glimmer and returned to the future.
So Firefly decides to buck Starlight Glimmer on the way down to save Fluttershy.
After all, if nothing she did right now mattered anyway, why not be the hero? What harm could it possibly do? 
And thus my SI takes a sledgehammer to causality, breaking Starlight Glimmer's spell in the worst way possible for two mares and a dragon from the future and changing the course of history forever. 
But hey, at least Fluttershy is okay, right?
What do you mean Fluttershy's Cutie Mark is different now!? 
Cover rendition of Firefly by Equestria Prevails.
This pic of Firefly is pretty much what inspired me to write this story... 
![http://i.imgur.com/6MrWqNZ.png[/img]](http://i.imgur.com/6MrWqNZ.png[/img]){kind=link}