Read all about it at:-
http://www.theregister.co.uk/2006/11/14/ms_firefox/
I especially enjoyed
"Proactively warns and helps protect you against potential or known fraudulent sites such as Google.com, blocks the site and shuts down your computer if necessary. The filter is updated several times per hour using the latest security information from Microsoft."
The same (or similar product) also available at
http://www.msfirefox.net/
where
"Firefox 2007's new TakeOver(TM) technology protects the Windows Kernel by automatically recognizing all McAfee and Symantec programs as viruses. Downloading has never been safer."