There are many extensions you can use which can work by auto detecting the IE setting for the website on FF.
The updated, fully supported IE Tab. Embed IE in a Firefox tab.
IE Tab Features: FF 4+ support, IE 7-9 compatibility modes, old IE Tab settings import. IE Tab supports ICBC and alipay
IE Tab Plus
The Original IE-Tab (Works also for IE8, IE9)
An enhanced version of IE Tab with Adblock Plus in IE supported. It also allows you import settings from IE Tab and synchronize cookies between IE and Firefox.
Only IE-Tab to support FF4 and beyond