The firebase CDN does not appear to have an API or a "latest" tag. The suggestion is:
<script src="https://cdn.firebase.com/js/client/2.2.9/firebase.js"></script>
A quick google shows that different sites have a lot of different version loading. I know I forget to update when I am forced to ask for a specific version.
The question is how can I infer the versions for firebase.js and firepad.(js|css) from a browser request?
Some examples...