Stack your tabs vertically on Firefox

This is a very useful Firefox Add-on: http://piro.sakura.ne.jp/xul/_treestyletab.html.en

The add-on allows you to stack your tabs vertically (instead of laying
them horizontally like a tool-bar). What makes it more useful is that
it maintains the tree structure for tabs. For example, if iGoogle
created a new tab when you clicked on a link on its page, it shows
that the tab has been spawned by iGoogle.

The add-on also allows you to collapse the tree view if you wish to see only the parent pages.