Session Buddy - Manage Your Browser Sessions
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.
Tabs Outliner - Next Generation Session, Windows, Tabs Manager and a TooManyTabs Solution That Really Works.
island - a free and opensource alternative to parellel space
Bulk URL Tool - With the Bulk URL Tool web application you can open up to 200 URLs all at once effortlessly.
Tree Style Tab - Tree Style Tab is an all-in-one Firefox extension that provides ability in order to operate tabs as a tree.