Thanks for the comments. This morning it allows me to add an exception. On Friday, with no change in certificate at all, it refuses me entry altogether.
Unfortunately, with a bug that's both rare and intermittent, I can imagine the developers could tear their hair out to resolve it but I think it is a Firefox bug. I may also raise it as a Zimbra bug for visibility.
On the days when it doesn't like me, I'm having to boot up Win XP in a Virtual Machine just to get access to IE7 to access the admin console |