Upgrade Firebase library usage from v9 compat to v9 modular

[dslucas]

We upgraded from v8 to v9 of the Firebase package in PR #5. v9 introduces a new modular API and accompanying methods, so we used the compat libraries to minimize the code changes. The compat libraries will be removed eventually, so we'll want to migrate accordingly.

See https://firebase.google.com/docs/web/modular-upgrade for more details.