This is one of the best Firefox extensions around and makes tabbed browsing even better than it already is. Tab Mix Plus can change the close button on each tab feature, control the rows of tabs when you exceed the width of the browser window, and almost everything else associated with tabs. Use your scrollwheel to browse tabs, or control tab switching via mouse gestures. You can specify where Web pages will open--in the background or the foreground, in a new tab or the current one--based on how they were created: by clicking a link, typing an address, or using the search bar.
You can duplicate a tab, protect it from closing, or merge tabs from several windows. We also like the option that prevents those annoying blank tabs when you download files. Another useful feature is a session manager that restores tabs that were open the last time the browser shut down. We'd like to see an option to view tabs as thumbnails, but nevertheless there's something here for every Firefox user--don't browse without it.