Microsoft is trying to lock out competing browsers when it comes to Windows running on ARM chips. IE is allowed there but not Firefox or Chrome or Opera or any other competitive browser. This is bad for the Web.
Here's what's going on. For Windows on X86, Microsoft is giving other browsers basically the same privileges it gives IE. It's not great that you don't get those privileges (certain API access) unless you're the default browser and I think that's deeply unfair (a post for later,) but at least we're able to build a competitive browser and ship it to Windows users on x86 chips.
But on ARM chips, Microsoft gives IE access special APIs absolutely necessary for building a modern browser that it won't give to other browsers so there's no way another browser can possibly compete with IE in terms of features or performance.
This is in direct violation of the promises they made to developers, users, and OEMs about browser choice in documents which mysteriously disappeared from Microsoft's site -- remember this? I sure do.
Here's a PDF of the pages that Microsoft disappeared in the run-up to their anti-choice decisions for the Windows OS running on ARM chips. Windows Principles - Empowering Choice, Opportunity, and Interoperability (PDF)
update: The Wall Street Journal has a blog post up titled Microsoft Accused of Hindering Firefox Browser and Stephen Shankland has posted Microsoft bans Firefox on ARM-based Windows, Mozilla says at CNET. The FT has also just posted Microsoft hit with complaint over Windows 8.
update 2:I've got more to say about this here.