The current button doesn't really resemble a tab, so it could be hard to guess. Something like IE's approach seems more intuitive. Still, a blank button seems l

The current button doesn't really resemble a tab, so it could be hard to guess. Something like IE's approach seems more intuitive. Still, a blank button seems like an analogy, not a direct message. A plus sign at the end of a tab row definitely means that by clicking it, you are adding something to it. And what else could it be, if there are only tabs in that row?