I agree with ESFP. I think Britney Spears is an ISFP just by the way she gets overwhelmed by her fame. I used to read teen magazines and I remember how in one of the interviews she said that she likes to be alone and needs her alone time. She's a very mildly expressed introvert, but I still believe she is one.
BTW, I've read her biography. As a kid, she willingly spent most of her time alone in her room practicing her dancing. She only became more social as an older teen, which corresponds to the introvert pattern of being more introverted as a kid and then becoming more extroverted as life goes on.
Just for comparison, Christina Aguilera is also an introvert.