My website is published on Google's Firebase Hosting. I own the domain. Google provision an SSL certificate (lately from Lets Encrypt, it used to be someone else). The certificate is shared between several domains using Subject Alternative Names. I don't own these other domains.
Once domain ownership is verified we will provision an SSL certificate for your domain and deploy it across our global CDN. This process can take several hours.
Domains will be listed as Subject Alternative Names in the FirebaseApp SSL certificate.
My question is: what would happen if a DNS hijack attack redirected requests to my domain to one of the other domains sharing the same certificate? A request to my domain would be received by one of the other serves sharing the certificate. How would the server respond? Would my browser accept the response? If not, how is this protected against?