Except that you can't run Android apps on your desktop out-of-the-box, so many people will prefer Window RT for that reason. Metro is essentially a Microsoft scheme to get lots of apps for Windows RT by leveraging Microsoft's monopoly position on the desktop.

Car analogy: So, if want to buy one of three cars, and the sales person walks up and keys the side of one of the cars, that's okay because I still have two other cars to chose from, right? That's what you're suggesting, that a diminished-but-not-nonexistent choice excuses a critical defect in one of the choices.

We aren't even talking about Windows 7 applications, though. This isn't a matter of Mozilla wanting to run legacy desktop code on Windows RT. Mozilla is being denied access to necessary APIs on Windows RT that would allow it to implement comparable functionality to IE10. Even if they chose not to implement desktop support in Windows RT (which they can't do anyways), the Metro version would be inferior because the operating system itself is designed to give IE10 access to higher security privileges, greater resources and more APIs. There is no level playing field to be had, even if you limit the field to the Metro UI.

If that was the case, IE10 Metro would be a dog, but that has not been the case for the Consumer Preview. WinRT is a very performant API, however if you've been writing programs in an unmanaged language then you're going to have to change, and this is something Mozilla doesn't want to do if they don't have to.

Actually, my understanding is that IE10 in not a pure Metro application. It uses both WinRT and Win32, and runs without many of the restrictions placed on Metro apps. Firefox for Metro uses the exact same model. This is a case of Internet Explorer on WindowRT having access to low level APIs that other applications don't. Third party browser vendors aren't competing on a level playing field.

UEFI firmware on Window RT tablets won't let you install any OS that the computer doesn't already have a signature for, and per Microsoft's certification requirements, you can't turn that security feature off, so unless the OS you plan to install has a certificate, and unless you tablet vendor allows installation of those certificates into your firmware, you can't run anything other than Windows RT.

For the average consumer who doesn't have a PhD in Computer Science, installing another OS is a pipe dream.

I think the locked down BIOS that only lets you run Window RT might also be an impediment...