What are some alternatives?
When comparing Tree Tabs and Tab Mix Plus, you can also consider the following products
Simple Tab Groups - Create, modify and quick change tab groups. Features:
Tabs Outliner - Next Generation Session, Windows, Tabs Manager and a TooManyTabs Solution That Really Works.
Simplified Tab Groups - This Firefox addon provides tab grouping features similar to the removed TabView/TabGroups/Panorama...
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.
Session Buddy - Manage Your Browser Sessions
Conex - TabGroups married with Tab Containers and bookmark & history search