IE Tab
If you use Firefox as your primary browser, you probably get stymied occasionally by websites that don't look right in its windows. Don't fire up Internet Explorer (IE), which maintains a separate history and won't have your bookmarks; instead, use the IE Tab plug-in to instruct Firefox to temporarily use IE's rendering engine. IE Tab sits in the bottom-right corner of your browser. Click the Firefox icon to switch to IE mode and vice versa.
Duplicate Tab
Want a copy of the current window you're visiting, complete with the history of that browsing session?
IE users can accomplish this without having to install and use a plug-in: press Ctrl, N to open a new window that shows the entire history of the current window.
For Firefox users, Duplicate Tab lets you obtain such a history with a single shortcut keystroke, or you can use the tool to merge multiple open windows into a series of tabs.
ErrorZilla
The standard 'Server not found' page is no help if you're looking for a website that's gone missing. ErrorZilla adds a series of buttons to the bottom of the standard Firefox message, providing instant access to the Wayback machine, Google Cache and more.
Inline Search
Internet Explorer users can obtain search-as-you-type functionality that works the same way it does in Firefox with this simple extension.
Extended Statusbar
The Extended Statusbar plug-in supplements the data that Firefox provides about a web page and your internet connection, providing such details as the total size (in KB) of the page, the transfer speed and the load time.