Sidetabs by Jeb Nicholson

Sidetabs adds 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 is the solution! See all of your tabs at once, better utilizing the horizontal space wasted by most modern mobile-first websites.

Sidetabs is designed as a drop-in replacement for Firefox's default tabs. Tabs look and feel the same, even supporting the same custom browser themes.

The extension is configurable, so you can customize how the tabs look and behave to better suit how you work.

In addition, Sidetabs provides a guided modification of your userChrome.css file, allowing additional features like autohiding tabs, removing the sidebar title and hiding the default tabs.