Mozilla is already locked out of iOS and they have failed at getting Apple to budge. If WinRT locks them out, they'll be locked out of two out of the three major tablet platforms (it's a safe assumption that WinRT will get some marketshare at least).
Microsoft is an easier target than AAPL - Win8 is still under development so it's still early days and the company has a history with various governmental organizations (the DoJ, the EU). My guess is Mozilla is attempting to rattle Microsoft into opening up Win8 to get Firefox to run. Or drum up enough press, get the DoJ to make a quote or two and really get Redmond to worry.
If I were Microsoft, I would hold my ground. Strategically, it makes a lot of sense to own the browsing experience e2e and I think Mozilla's case is weak given MSFT's lack of marketshare, the existence of Chromebooks, Boot2Gecko, etc.