Both ctrl-click and (for newer Mac laptops) two-finger-click should work. BUT Firefox is a little wonky on Mac with Zimbra. Ctrl-click actually has an ambiguous meaning because, in Zimbra, it's also used for multiple selection. So sometimes you have to click more than once.
Try it with Safari, you'll see that Zimbra behaves more nicely.
There's an entry in bugzilla about this:
Bug 35543 – control key behavior clashes with Macintosh control-click operation