Features:

• You can collapse/expand sub trees. It is convenient for too many tabs.
• When you close a tab which has collapsed sub tree, all of tabs in the sub tree will be closed only one action.
• The tree of tabs can be showin at rightside.
• Vertical tab bar cab be shown/hidden automatically.
• You can open new tab between existing tabs, dropping link or URL string there.
• New child tab can be opened automatically from the location bar, if the website is same to the current tab's. Of course, different website can be opened in new tab too.
• A link can be loaded in new child tab automatically if it is to a different website. If you wish, you can load any link in new child tab.
• You can save/restore the tree of tabs over sessions, by Session Manager or other session-management addons.
• Drag and drop is available to modify relations of tabs and to rearrange. The behavior seems to be like "layer tree " of Adobe Illustrator/Photoshop.
• If you focus to the tab bar, Ctrl-Up/Down/Right/Left can be used to modify tree.
• Tab bar can bemoved to leftside, rightside, top, or bottom. Horizontal tree is also available.
• This has an API for extension authors to open child tabs for existing tabs.