What are some alternatives?
When comparing Tree Tabs and Tabs Outliner, you can also consider the following products
Simple Tab Groups - Create, modify and quick change tab groups. Features:
OneTab - Whenever you find yourself with too many tabs, click the OneTab icon to convert all of your tabs into a list. When you need to access the tabs again, you can either restore them individually or all at once.
Simplified Tab Groups - This Firefox addon provides tab grouping features similar to the removed TabView/TabGroups/Panorama...
Session Buddy - Manage Your Browser Sessions
Conex - TabGroups married with Tab Containers and bookmark & history search
Session Box - Using websites with multiple accounts at the same time is made easy.