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.

Except...

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.)

The only way I can get it to stop doing the perpetual reloading is to do a full refresh in the browser window. The entire page reloads and it's fine. (until it decides to start reloading again...) The Firefox javascript (Error) console doesn't show any errors during this time.

This is with ZCS 5.0 RC2, Firefox 2.0.0.11, Mac OS X 10.4.11...