Firefox Sync is a service that lets you synchronize your bookmarks, history, passwords and open tabs with another copy of Firefox – like one on another computer or on a mobile phone. In previous versions of Firefox, you needed an extension for this. Now Sync is part of Firefox (since Firefox version 4), and is very easy to set up. I use Sync to synchronize bookmarks between my work laptop and my regular and admin accounts at my home laptop.
Tab Mix Plus enhances Firefox's tab browsing capabilities. It includes such features as duplicating tabs, controlling tab focus, tab clicking options, undo closed tabs and windows, plus much more.
It also includes a full-featured session manager with crash recovery that can save and restore combination of opened tabs and windows.
My favorite settings for this extension:
Mouse double-click on the tab: duplicates the tab.
Show in tab context menu: new tab; merge windows; reload all tabs; undo closed tabs; closed tabs list; close other tabs; bookmark all tabs.
Show in main context menu: open links in new tabs.