I have seen some similar posts, but none with exactly this problem.
I am running Win7 enterprise 64bit, Firefox 3.6.8 and Zimbra CS (can't see how to find a version number).
In calendar view, when I right-click on an icon, I get the Windows context menu coming up on top of the Zimbra contect menu!
I didn't realise this for ages, I thought the Zimbra menu wasn't coming up, but if I click exactly on one edge of the top menu, up comes the Zimbra menu.
However this is not a long-term solution. Does anyone know if this is a Firefox or a Zimbra problem?