Mozilla has come forth with a big reveal that Windows RT (Formerly Windows 8 on ARM) will only support a single browser: Internet Explorer. From a technical side Firefox would be able to run in Metro mode on Windows RT, but would be crippled to the point of unusable. Only Office 15 is allowed to run in the classic Desktop Mode with Windows RT.
The issue has to do with API access. Third-party developers will only have access to the WinRT (Metro) API. However, Microsoft's software will have access to the lower level Win32 API. Firefox could build Firefox for ARM but without the ability to tap into the Win32 API it can't compete with Internet Explorer.
Some of the logical reasons behind this move by Microsoft is that they have not wanted to port x86 code to ARM, largely because these ported apps may not have the efficiency and stability of a low-power ARM tablet requirement.
Another reason could be due to malware attacks. Since a browser is an open pit for malware attacks on the operating system making it so there is only a more secure Metro Internet Explorer could be a protection mechanism.
The Tech-Stew Take Home
This move by Microsoft seems akin to the early dark days of the 90s and possibly a move to gain market share with Internet Explorer, at least on the surface. Hopefully the primary motivation here is that they are just trying to protect the operating system and by association the end-users giving them a more pleasant experience with less risk of malware attacks.
The downside is that due to this restriction it means we won't see any browsers with add-ons on Windows ARM tablets. Nor will there be any HTML5 web apps. Competition gets squashed as a result and the chance of ARM tablets winning out over x86 tablets becomes less likely.