http://bugzilla.zimbra.com/show_bug.cgi?id=24231
In both 5.0.1 and 5.0.2, I can reproduce this behavior reliably on firefox 2 for the mac (only) if the following conditions are true at the time of zwc startup (i.e., at login or reload with different skin):
1) zimbraPrefForwardReplyInOriginalFormat=TRUE (can only be set in admin console or zmprov)
2) zimbraPrefComposeFormat=html
3) Reply to a text/plain message, which overrides the general html pref and uses the text composer. It's like zwc is expecting the browser html design mode to start, but it never does. Bug 24231 and your experience show it's a more general problem.
In both the logged bug and my experience, there is no browser-server traffic. It's all at the client
JS level.