The main reason you might want to use Firepad instead of Etherpad is if you don't want to run the server-side piece of Etherpad. For example, if you're stack is different than theirs it might be a pain to set up a special server environment just for the collaboration portion of your app.

Firepad is fully client-side code, so there's no server software for you to set up.

The other big reason you might use Firepad is to get all of the other goodies that Firebase provides, like the automatic scaling, Forge debugger, declarative security, etc.