I have a support case open on this, but wanted to cast a wider net to make sure this can be reproduced elsewhere.
In class of service, set zimbraPrefComposeFormat text. It's faster and more compatible.
In class of service, set zimbraPrefForwardReplyInOriginalFormat TRUE (this might be a new option for 5.0). The reason we set this is that users were complaining that HTML formatting was lost when forwarding formatted messages received from offsite. Try to forward an HTML-formatted newsletter as text, and you'll be disappointed. As far as I can tell, ZWC provides no way to set this for individual users to set this option.
For the individual user, set zimbraPrefComposeFormat html. Some users have no taste.
In Firefox for Mac *only*, when a user configured as above replies to or forwards a text-only message, the zimbraPrefForwardReplyInOriginalFormat TRUE CoS setting overrides her zimbraPrefComposeFormat html setting, and the text composer is used. But something gets very unhappy about that. firefox-bin eats 100% of CPU, and the user notices tabs reloading, and things getting really slow. The only way to break the cycle is to kill the AJAX client, either by closing the window or clicking "Switch to Standard (HTML) version."