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.

Tabs Outliner - Next Generation Session, Windows, Tabs Manager and a TooManyTabs Solution That Really Works.

Toby - Better Than Bookmarks

Tree Style Tab - Tree Style Tab is an all-in-one Firefox extension that provides ability in order to operate tabs as a tree.

Session Buddy - Manage Your Browser Sessions

Tab Utilities - Tab Utilities is an ultimate tab solution for all kind of users who want to manage their tab, create multiple session and search for new tabs, etc.