Sidetabs by Jeb Nicholson

Sidetabs adds the the vertical tabs option that Firefox never had.

Tired of opening hundreds of tabs, and then not being able to find the one you want? Vertical tabs fixes this issue by showing all of your tabs fully, allowing you to quickly scan them for the one you need.

Sidetabs is designed as a replacement for Firefox's regular, horizontal tabs. It looks and feels just like the default, even supporting custom browser themes.

The extension is configurable, so you can customize the look and feel of Sidetabs to suit how you work.

In addition, Sidetabs provides a guided modification of your userChrome.css file in order to support additional features like autohiding tabs, removing the sidebar title and hiding the horizontal tabs.