I've been having some "beachball of death" issues on my desktop Mac so did some investigating.

Activity monitor said Firefox was fluctuating between 7-60 % CPU, even when idle. I normally open Firefox with 10 tabs to various websites, one of which is developer.anscamobile.com. If I close Ansca's tab Firefox's CPU usuage goes down to a steady 3-4 % CPU.

Not sure what Firefox doesn't like about your webpage, but my suspicion is the frenetically updating "tweet" window. Seems like even that shouldn't be that expensive, though.