Vertigo

No, not that dizzy, spinning sensation. The Firefox extension.

Someone finally caught on to the fact that on a lot of modern computers, we have a lot more horizontal space than vertical. Whoever it was, they must have also been miffed that they never seemed to have room for all of their firefox tabs. So, they made a vertical tab menu, which is actually pretty cool. The standard stuff (ctrl-tab) still works, which is convenient.