Se estas são as únicas colunas com as quais você está preocupado, isso funcionará para você:
SELECT um.user_id
, fn.meta_data AS first_name
, ln.meta_data AS last_name
, e.meta_data AS email
FROM wp_userMeta AS um
LEFT JOIN wp_user_Meta AS fn ON um.user_id = fn.user_id
AND fn.meta_key = 'first_name'
LEFT JOIN wp_user_Meta AS ln ON um.user_id = ln.user_id
AND ln.meta_key = 'last_name'
LEFT JOIN wp_user_Meta AS e ON um.user_id = e.user_id
AND e.meta_key = 'email'
