I have solve issue by adding _address_firstname
and _address_lastname in header.
in my file missing above column and so its display error of firstname is invalid.
Below is header value using space separated and its working now.
email _website _store store_id created_in website_id group_id firstname lastname _address_telephone _address_postcode _address_country_id _address_region _address_street _address_city taxvat _address_company _address_default_billing_ _address_default_shipping_ _address_vat_id _address_firstname _address_lastname