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.
Monitors web pages for updates. Useful for websites that don't provide Atom or RSS feeds.
Makes your Bookmarks Toolbar appear in multiple rows.
For Firefox 4 use: Multirow Bookmarks Toolbar Plus, available here:
New Tab King is the premier extension for turning your new tab page into a beacon of productivity and efficiency.
Featuring a todo list, recently closed tabs, most visited pages and more!
Tons of visual customization option too!
ShareThis for FireFox makes sharing simple. Share anything on the web with your friends on 45+ social sites including Facebook, Twitter, Email, Digg, Reddit and more. Track top trending shares by site and automatically save content to your ShareBox.
Mobile Barcoder generates 2D barcodes of URLs and text selections so that you can transfer that information to your mobile easily.
OPML Support adds OPML import/export functionality to the Firefox Bookmarks manager.
Bookmark Previews adds an album view and thumbnail view to the bookmarks manager.