Yah. Thats true. But not the kind of add-ons that you would love to install in your firefox! These are for WGA validation extensions for Office 2003 and Windows. Earlier, for Firefox users, MS gave a small stand-alone application for genuine validation. Now they have created proper plugins.
I came across these interesting plugins when I tried to download some add-on applications for Outlook 2003. Type about:plugins on your firefox address bar to see if you have any of these plugins installed.