I have had Zimbra installed and running flawlessly for 18 months in a small business environment. It is 4.0.3_GA_407 running on Ubuntu. The users access Zimbra using Firefox running on Windows XP from their desktop PCs. Recently a couple of the users updated to Firefox 3.0.1 and since then they have not been able to view calendar items. They can still add items to the calendar but are unable to view the new entries (or the entries that were there before they updated firefox). I have uninstalled all Firefox addons (there was only Adblock Plus). They can view the calendar entries from the same PC using Internet Explorer 6.0.2900.2180 and can view their calendar entries from a different PC using Firefox 188.8.131.52. So I'm pretty sure it is a Firefox issue.
Any ideas ??