I have been using the FF browser for like last 5 years, but this is the first time I observed this feature, that too accidentally. If you double click on the empty space on the bar next to your browser tabs, a new tab opens up.

There is no indication anywhere that a double click will open up a new blank tab. That is one kool thing I found in IE browser, that small box next to your last browser tab, which very clearly tells you “click me! I will open up a new tab”.

I guess that is what we call usability.