I agree with the people who say it's not related to type.
Out of the NFs I know, it seems to average out at about medium self esteem. There's one INFJ who's self-esteem is ridiculously low, one ENFP with fairly low self-esteem, and one INFP with really high self-esteem, but most of them fall somewhere in the middle.
I'd say the corrolation is more between perceived self-esteem and type, like NFs might appear to have low self-esteem when really they're trying to be modest. Kind of the opposite sort of thing as the NT's perceived confidance.