Britney, if she releases her single around December, which will most likely happen cause the album is getting released in March. No one else has the selling power Britney has and others got no chance at all, especially if she bothers to do some promo this time.
I don't think "Firework" will hit #1 (although I would love for that to happen) and I don't think Ke$ha will climb back to #1 again.