I have been using zimbra now for a few weeks. I primarily use FireFox, without issue. It is fast and I have never had any issues. One of my users just today used IE and when they login they get the following error:
msg - Csfe service error
code - NETWORK_ERROR
method - ZmCsfeCommand.prototype.invoke
detail - Empty HTTP response
There is nothing in the zimbra.log file. The server is not overloaded. It is a Dual Zeon 3.0 Dual Core with 2G of memory. The OS is on Raid 1 and the /opt/zimbra partition is on Raid 1 + 0. We have about 15 users that mainly use outlook to get email.