What are some alternatives?
When comparing Tree Tabs and Simple Tab Groups, you can also consider the following products
Simplified Tab Groups - This Firefox addon provides tab grouping features similar to the removed TabView/TabGroups/Panorama...
Tabs Outliner - Next Generation Session, Windows, Tabs Manager and a TooManyTabs Solution That Really Works.
Conex - TabGroups married with Tab Containers and bookmark & history search
Tree Style Tab - Tree Style Tab is an all-in-one Firefox extension that provides ability in order to operate tabs as a tree.
Sidewise Tree Style Tabs - A dockable sidebar for Chrome featuring vertical Tree Style Tabs, Tab Hibernation, and much more.
Tab Workspaces - Organize your tabs into workspaces.