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The upcoming release of Windows 8 will include a version that expands Microsoft’s operating system to tablets that use the ARM architecture common in mobile devices. It’s part of Microsoft’s attempt to compete against the iPad.

But Firefox maker Mozilla contends that it will also give Internet Explorer an unfair edge over rival web browsers.

In Windows RT, the name of the version of Windows 8 for ARM devices, Firefox will be able to offer a Metro app, using the new tile-based design and development model that Microsoft is  introducing wit the new operating system. “However, Windows on ARM prohibits any browser except for Internet Explorer from running in the privileged ‘Windows Classic’ environment,” writes Harvey Anderson, Mozilla general counsel in a blog post.

He continues, “In practice, this means that only Internet Explorer will be able to perform many of the advanced computing functions vital to modern browsers in terms of speed, stability, and security to which users have grown accustomed. Given that IE can run in Windows on ARM, there is no technical reason to conclude other browsers can’t do the same.”

The situation will be different for Windows 8 on traditional x86 machines, with an approach that gives Metro-style browsers the ability to bridge the old and new environments. PCWorld has a good explanation of the technical aspects situation.

Anderson says the situation has antitrust implications. Microsoft has agreed to abide by a set of principles to maintain a level playing field on Windows for competitors despite the expiration of its consent decree with the U.S. Justice Department.

However, while the company dominates the traditional PC market, it’s starting from scratch on ARM devices, and it’s not clear how regulators would define the relevant market, which is one of the key issues in antitrust cases.

Microsoft hasn’t yet commented publicly on the Firefox post, but we’ve asked the company for a statement, and we’ll update this post if we get one.

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