Membership.GetAllUsers() returns a MembershipUserCollection, which you can use to access individual MembershipUser. Example:
MembershipUserCollection users = Membership.GetAllUsers();
string email = users["some_username"].Email;
You can also retrieve ProfileInfo in the similar way:
ProfileInfoCollection profiles = ProfileManager.GetAllProfiles(ProfileAuthenticationOption.All);
DateTime lastActivity = profiles["some_username"].LastActivityDate;
However there are no FirstName and LastName properties by default, unless you manually specified them in your profile provider.
Check out MembershipUser class and ProfileInfo class for more details. You might also wanna check out SqlProfileProvider class as an example of profile provider, unless you already have implemented one.