For my personal site, I have a firebase project (using firebase hosting) which is alloted to my apex domain (http://exmaple.com), and I have another firebase project, which I want to allot to a subdomain of the same apex domain (http://subdomain.example.com), but firebase doesn't allow me to do that, any workarounds?
"This is currently a limitation of how custom domains work on Firebase Hosting. We're exploring ways to improve and make it possible to reuse the same apex domain across multiple sites, but we don't yet have anything concrete to announce at this time." - Michael Bleigh