Set the Firestore config documents through the console or in default-firebase-data.json before initial data boostrap.
{
"config": {
"rendertron": {
"server": "https://render-tron.appspot.com"
},
"site": {
"domain": "YOUR_SUB_DOMAIN_HERE.web.app"
}
}
}https://render-tron.appspot.com is provided without uptime guarantees so you may want to deploy your own version.