Workaround https://github.com/odoo/odoo/issues/6324.

partner_firstname/views/res_user.xml

@@ -2,6 +2,10 @@
 <openerp>
 <data>
 
+<!-- Required before modifying `base.vew_users_form`.
+     https://github.com/odoo/odoo/issues/6324#issuecomment-93534579 -->
+<function model="res.groups" name="update_user_groups_view" />
+
 <record id="view_users_form" model="ir.ui.view">
     <field name="name">Add firstname and surnames</field>
     <field name="model">res.users</field>