Hi Jeffrey,
Thanks for submitting this suggestion.
It sounds like you are suggesting an ability for clients to register or sign in to the client area using their Firebase account credentials, is that correct?
It's my understanding that one accessing Firebase using a Google account. WHMCS already supports Google as a Sign-In Integration module: https://docs.whmcs.com/Sign-In_Integrations

Please do provide more details if that isn't quite what you're suggesting here.