Sidebery - Manage your tabs and bookmarks in sidebar.
SplitUp! - A fun and elegant tab manager
Tree Style Tab - Tree Style Tab is an all-in-one Firefox extension that provides ability in order to operate tabs as a tree.
Tab Stash - Tab Stash is a browser extension that helps you keep your tabs straight, saved, and restore them later.
Firefox Multi-account Containers - Sign in to two different accounts on the same site
Smart Tab Manager - Smart Tab Manager is a browser extension that allows you to save & restore your browser sessions.