social/website_livechat_firstname/views/website_livechat_firstname_view.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright 2017 Specialty Medical Drugstore
     License LGPL-3.0 or later (http://www.gnu.org/licenses/lgpl). -->

<odoo>

    <template id="channel_page" inherit_id="website_livechat.channel_page" >
        <xpath expr="//div[@class='media-body']/h5/t" position="replace">
            <t t-if="user.partner_id.firstname" t-esc="user.partner_id.firstname" />
            <t t-else="" t-esc="user.partner_id.name" />
        </xpath>
    </template>

</odoo>