Mozilla issued a statement Wednesday expressing concern about some of the technical restrictions that Microsoft is imposing on its ARM port of Windows 8. Microsoft’s policies will effectively prevent Mozilla and others from bringing their Web browsers to the Windows 8 desktop on ARM systems.

Microsoft originally announced plans for Windows on ARM (WOA) at the Consumer Electronics Show (CES) in 2011. The ARM architecture will be fully supported in Windows 8 through a special version of the operating system called Windows RT. The ARM port will emphasize the use of Microsoft’s distinctive Metro environment.

Unlike the x86 flavor, however, WOA will not broadly support legacy applications. WOA will only run applications that are distributed through Microsoft’s application store. Third-party developers who bring their software to WOA will be confined to using the Windows Runtime stack and standard platform APIs. As Microsoft explained in a February blog post, the point of these restrictions are to maximize security, performance, and battery life for WOA users.

“If we enabled the broad porting of existing code we would fail to deliver on our commitment to longer battery life, predictable performance, and especially a reliable experience over time. The conventions used by today’s Windows apps do not necessarily provide this,” Microsoft’s Steven Sinofsky wrote. “By avoiding these constructs, WOA can deliver on a new level of customer satisfaction: your WOA PC will continue to perform well over time as apps are isolated from the system and each other, and you will remain in control of what additional software is running on your behalf, all while letting the capabilities of diverse hardware shine through.”

Microsoft’s approach is consistent with that of other platform vendors who are targeting the ARM architecture, including Apple. Requiring third-party developers to use a standard, isolated stack allows the platform to have tighter control over things like application lifecycle, ensuring that rogue software doesn’t abuse the device’s limited hardware resources.

The downside of this approach is that it makes it difficult to support certain kinds of highly complex third-party software, such as Web browsers, that require more capabilities than those provided by the standard API set.

The consequence for WOA users is that they will only have access to Internet Explorer, Microsoft’s own Web browser, in the desktop mode. Mozilla General Counsel Harvey Anderson suggested in a statement that this policy decision by Microsoft “restricts user choice, reduces competition, and chills innovation.”

The move also contradicts Microsoft’s Windows Principles, Anderson said. The principles are a set of guidelines that Microsoft drafted in 2006 as governance policies for Windows development. At the time, Microsoft committed to ensuring that users always have the option of using third-party software as their default application for key features, such as media playback and Web browsing. Anderson also suggests that there may be “antitrust implications” if policy is found to fall afoul of Microsoft’s commitments in Europe and the United States.

“We encourage Microsoft to remain firm on its user choice principles. Excluding 3rd party browsers contradicts Microsoft’s own published Principles that users and developers have relied upon for years,” Anderson wrote. “The prospect that the next generation of Windows on ARM devices would limit users to one browser is untenable and represents a first step toward a new platform lock-in. It doesn’t have to be this way.”

Google, maker of the popular Chrome browser, is echoing Mozilla's concerns. "We share the concerns Mozilla has raised regarding the Windows 8 environment restricting user choice and innovation," a company spokesperson said in a statement. "We've always welcomed innovation in the browser space across all platforms and strongly believe that having great competitors makes us all work harder. In the end, consumers and developers benefit the most from robust competition."

The policies that Microsoft has established for Windows RT seem reasonable today when considered in the context of how the platform is intended to be used. Demanding support for Firefox on Windows RT is a lot like asking for Opera on a Chromebook or WebKit on Mozilla’s own Boot2Gecko platform—it would conflict with the underlying purpose of the platform.

But the issue raises some difficult questions. As Anderson highlighted in his statement, Windows RT has the potential to see adoption on a wide range of additional form factors beyond tablets, including systems that are more like traditional PCs. In such cases, the exclusionary aspects of the platform could prove problematic for users and developers.