Well, it's secure as long as you're the only one using the computer, or witnessing its use by other person, or you protect them with a master password. The file containing the passwords is encrypted with the Triple DES algorithm (in Opera, and I think in Firefox as well), so no problem there.
Don't know about the scheme used by IE, anyway, one of the advantages of storing the passwords in the browser is taking advantage of the portability most browsers offer, and IE is not one of them :)
EDIT: Removed confusing abbreviation (or something)