Yeah, we just launched this today and are in very early stages of beta. We realize speed is an issue right now (we've mostly been testing on Windows and OS X, on which it runs better), and it's definitely something we're working to improve.
Yet:
"Powered by Cappuccino: no Flash slowing down your browser."
Point taken. We changed it.
Are you using an old firefox version by any chance (<3.5.x)?