The only way to guarantee information security is to hand roll your own encryption on the server. You could host your firebase connectivity server-side and have your user send the data to that via SSL and from there do your encryption and then use the SSL address of firebase to store.
On the clientside, things are suspect to CSS attacks. If you really want to go down this route you can use js encryption from this lib: http://code.google.com/p/crypto-js/. Note that crpto-js works well in isolation but you will also need to be sure your webpages are not tampered with (quite hard to do IMOP, cause you don't know whats infected the users machine)