Clearly this is a hoax, but someone mashed up the Microsoft Internet Explorer home page with that of Mozilla Firefox to create the MS Firefox page. It's not too far fetched; Mozilla Firefox is open-source software and already Hacktivismo has based their on Mozilla Firefox 1.5, and Microsoft recently said it is exploring open-source solutions--but this is too much. For example, under the Security tab, the MS Firefox site says: "A robust new Microsoft security architecture known as TakeOver helps to protect the Windows Kernel from malicious, damaging and viral software such as Symantec and McAfee products."
There's even a parody video about how life is much better with Microsoft Firefox 2007 Professional Edition and a fake press release stating that Microsoft has acquired Mozilla Firefox. If you're in for a good laugh, check out the various pages.