Joel Spolsky has his My three favorite Firefox extensions, very cool. I specially love the first two, IETab and Web Developer Extension.
IETab
IE Tab, an extension from Taiwan, embeds Internet Explorer in a Mozilla/Firefox tab.
This extension is derived from the famous extension IE View, but much different.
While IE View always open IE-only pages in a seperate Internet Explorer window, IE Tab can view them in a Mozilla/Firefox tab.
Web Develop Extension
The Web Developer extension adds a menu and a toolbar to the browser with various web developer tools. It is designed for Firefox, Flock, Mozilla and Seamonkey, and will run on any platform that these browsers support including Windows, Mac OS X and Linux.
If you are viewing this page in FireFox, you can install them right from below:
IE Tab
it's true, you can run Windows Updates right from your FireFox browser, isn't it nice??
Web Develop Extension
compare to IE developer toolbar, this seems to offer more features for web developers