Because letting in FireFox means letting in Chrome which means letting in Google to entirely subvert their beautiful new OS and turn it into ChromeOS. MS needs

Because letting in FireFox means letting in Chrome which means letting in Google to entirely subvert their beautiful new OS and turn it into ChromeOS. MS needs to be in a position - just like Apple - to ensure that browser based apps remain at least just slightly worse than native ones so they keep a chokehold on the tollgates to the river of digital money that will come flowing through their app store.