为什么FireBase使用自定义域以外的其他域发出SSL？

I have deployed my app for about 3 months now, but I'm still seeing different SSL certificate. Also, it seem that it has renewed it using different domain. Is this still normal? If yes then how long does it normally take to complete a provisioning? Is there a way use my own certificate instead? ... Since it's just using LetsEncrypt after all.I have deployed my app for about 3 months now,