There was an earlier extension, IE View but this one is different from that. IE View always open IE-only pages in newly launched windows of Internet Explorer, IE Tab opens them in tabs of Mozilla/Firefox.
Personally, why do I feel it is really cool?
Well, it is like having Internet Explorer inside Firefox. Most of the banks I deal with (ICICI, HDFC, Citibank, Standard Chartered), Phone Sites (Hutch, Airtel) and Movie Theatre Sites (Fun Republic, Fame Adlabs, IMAX) sucks on Firefox (sometimes, don’t work at all) and this Extension have been a boon for me.