Moxilla's FireFox team defines an extension as:
"Extensions are small add-ons that add new functionality to Firefox, from a simple toolbar button to a completely new feature. They allow you to customize Firefox to fit your own needs and preferences, while letting us keep Firefox itself light and lean."
Other types of add-ons for FireFox include custom themes or skins, plugins for things like Flash and Java and search engines of all types to add to the search window at the top of FireFox.
IE Tab 1.3.0.20070110 by PCMan (Hong Jen Yee), yuoo2k
IE Tab - an extension from Taiwan, features: Embedding Internet Explorer in tabs of Mozilla/Firefox.
This is a great tool for web developers, since you can easily see how your webpage displayed in IE with just one click and then switch back to Firefox.
If you need some instructions.
Please visit our official website on http://ietab.mozdev.org