Tree Style Tab - Tree Style Tab is an all-in-one Firefox extension that provides ability in order to operate tabs as a tree.
Lightbeam for Firefox - Mozilla is the not-for-profit behind the lightning fast Firefox browser. We put people over profit to give everyone more power online.
Simple Tab Groups - Create, modify and quick change tab groups. Features:
island - a free and opensource alternative to parellel space
Sidebery - Manage your tabs and bookmarks in sidebar.
Tabs Outliner - Next Generation Session, Windows, Tabs Manager and a TooManyTabs Solution That Really Works.