I work with Zimbra trough web mail interface on Firefox.
I've got a strange issue upgrading my FF to 3.5.x: the logon window was very quick to appear, but then it was terribly slow to show all Zimbra window and I could not be able to read any messages, even they was correctly listed.
I did try to remove and reinstall FF several times (cleaning windows registry too...), create a new user, clean FF cache-history with no success. Then I resolved uninstalling Skype component in Tools -> Components -> Extensions of my FF. That component is built-in in the 3.5.x setup. Hope this helps.