I'm going out on a limb here, but I just reread your original post and have another idea. In the past, I have seen Firefox do weird things (like, blank screen weird things) when I tried to access a secure site (such as a webmin page) for some site where the hostname and IP were the same, but the certificate had changed due to a restore or upgrade of software. This has happened on my firewall as well as Zimbra, I think. You might try going into your certificate stores (and root certs path) on your client machine, and deleting all certs for your Zimbra box and/or its IP, so that next time you try to access the UI you have to re-load the cert. It's a long shot but it might do the trick.