I
So I have spent too much time on this. There isn't really a more elegant solution than onMOunt.
However, I did realize that sapper really should be used for it's SSR capabilities. And I wrote an article about how to get set up on Firebase with Sapper SSR and Cloud Functions:
https://dev.to/eckhardtd/how-to-host-a-sapper-js-ssr-app-on-firebase-hmb
Another solution to original question is to put the Firebase CDN's in the global scope via the src/template.html
file.
%sapper.html%
%sapper.scripts%