A bit of a surprise to her parents, who had never really planned on a foal. Still, Firefly and Pyro did all they could for their new filly, naming the little unicorn after Firefly's beloved grandmother, Firecracker Spark. To this day, they've never come up with a reason as to why they went with 'Burst' for their filly, but she doesn't mind.
Firecracker Burst grew fast, and as with most unicorn foals, her magic showed up in powerful early surges- and in her case, destructive. It was clear early on that the filly was going to have a skill when it came to fire, as more often than not, any little waves of magic resulted in a blanket, toy, or (in one case) even her crib becoming quite charred. Thankfully, Pyro's fire-containment talent was enough to reduce most risk, and things had settled down once it was time for her to begin school. There weren't a lot of magic classes, San Celestia not really having an enormous unicorn population, so young Firecracker spent most of her spare time practicing whatever spells she could out of the old, second-hoof spellbook her father had picked up. She loved the fire spells, but nothing ever felt... right. Oh, the flames were impressive, she even had a knack for producing fire of different colors, but there was no... oomph.
That urge to find something bigger and better may have contributed to the unusually long time it took for Firecracker to earn her cutie mark- in fact, it wasn't until her second-to-last year of school that she finally got it! She had long enjoyed chemistry classes, and as their final project for that year, each pony was allowed to put what they'd learned to use. Some baked, citing the exact reactions between ingredients; some did presentations on rocks... but Firecracker did something big. She had become fascinated with the exact mechanics of firework displays, and on the final night of the school year, she presented fireworks of her own. And how proud was she! The show went off without a hitch, and while she still had a lot to learn, the brilliant colors and sound and excitement were exactly what she had been looking for her entire life. Once the display ended, there was a burst of applause- not just for the show, but for the fact that their classmate had finally gotten her cutie mark!
Since then, Firecracker Burst has spent her time refining her firework techniques, as well as her various fire spells. Her work with entertainment eventually led her to getting involved with planning all sorts of events during the off-season, which she enjoys nearly as much as her main line of work.