I have been able to reproduce the problem (though I cannot at this moment...), but cannot determine what's actually going on behind the scenes.
When I am in my Inbox, click a message and then click Reply All. The compose div is displayed and everything seems to work correctly.
The browser starts doing a background reload constantly.
It must be part of the Ajax, it drives the CPU to 100% and never stops on it's own, regardless of any actions I attempt. (It doesn't change when viewing other tabs, performing actions on calendar items, addresses, etc.)
This is with ZCS 5.0 RC2, Firefox 18.104.22.168, Mac OS X 10.4.11...