​​Tree Style Tab

This extension for Firefox provides the ability to work with tabs as with "trees".

New tabs opened from the current tab are automatically organized as "children" of the current tab. 

Such "branches" are easily collapsed by clicking on the arrow shown on the "parent" tab, so you no longer need to suffer from too many visible tabs.

If you want, you can change the tree structure by dragging and dropping.

Tree Style Tab also provides an API for other extensions. Some extend the behavior of the Tree Style Tab sidebar.

Firefox-addons: