This is nothing more than spin. The Windows 8 platform is used by virtually the same number of users that Windows 7 had at this point in its release. The problem for Mozilla is most Windows 8 users are simply using the preinstalled IE browser, or are using Chrome for those not using the Metro interface.
Has nothing to do with the platform. Mozilla has been losing marketshare to Google for years, and since Windows 8, IE has seen a dramatic increase in users as well.
Opera is a better browser than all of them.