It's not that there are more than 50 different Android tablets to choose from.
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.
Is MS really forcing you to buy Windows RT?
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.
You could make that case on the desktop for apps, but exactly zero apps that run on Windows 7 will run on Windows RT.
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.