What are some alternatives?
When comparing UndoClose and Session Box, you can also consider the following products
Tree Style Tab - Tree Style Tab is an all-in-one Firefox extension that provides ability in order to operate tabs as a tree.
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.
Undo Close Tab - Reopens the last closed tab.
Session Buddy - Manage Your Browser Sessions
MySessions - MySessions is a simple session manager.