Why should things like this exist?! Rated 2 out of 5 stars
Personally, I think the idea of IE on FF is stupid, because web developers should care less what browser someone uses... in a "fair and unbiased world" they wouldn't care and would be more concerned with there OWN content. But they do.
Take Microsofts's skydrive site (go figure -- it's Microsofts). For uploading files to the sky drive on FF, you can only upload up to 5 at a time, and you must manually browser for each file, pretty time consuming. BUT with IE you can use the activeX control and drag and drop as many files as you want into "drop box" for you upload, and you can actually see the progress of each file. Clear this is just Microsoft touting how awesome its ActiveX. BTW on FF, you get a flash app that lets you bounce ball while you wait. Im sorry, but I'm not a monkey.
This extension gives you both, you can embed IE into FF and get the advantages (and more importantly, the disadvantages) of running activeX on FF and other IE unique features.
While i have a lot of beef with poor website developers developing sites for different browsers, this extension is designed incredibly well. I gave it 3 stars because well, anything involving IE is less than perfect. Also, it is my opinion extensions like this should not be encouraged, we should be supporting open standards. And lastly, you really are using IE when you use this, that means history, cache, cookies.... security issues.... activeX... scripts... everything that's bad about IE can and will leak through.
Most sites now are not browser sensitive, including Microsoft's... so think it over.
Lastly, other FF extensions don't work while in the IE mode (Fasterfox, Refcontrol) and its worth noting history items may appear in IE's history and not FF (cookies, cahce, history, etc.).