What are some alternatives?
When comparing Open Tabs and Tree Tabs, you can also consider the following products
Tabs Outliner - Next Generation Session, Windows, Tabs Manager and a TooManyTabs Solution That Really Works.
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.
Simple Tab Groups - Create, modify and quick change tab groups. Features:
Session Box - Using websites with multiple accounts at the same time is made easy.
Simplified Tab Groups - This Firefox addon provides tab grouping features similar to the removed TabView/TabGroups/Panorama...
Tab Mix Plus - Tab Mix Plus enhances Firefox's tab browsing capabilities.