Mozilla and Google are crying foul over Microsoft restrictions blocking rivals from Windows 8 on ARM, due later this year.

Firefox-shop Mozilla has branded Microsoft's restrictions a return to the digital dark ages "where users and developers didn't have browser choices".

Harvey Anderson, Mozilla general counsel, accused Microsoft of restricting user choice, reducing competition and chilling innovation by only allowing Internet Explorer to run on Windows RT – unveiled last month by Microsoft as the new name for Windows on ARM (WOA). He said:

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.

Mozilla Firefox director Asa Dotzler has weighed in with the technical argument:

On ARM chips, Microsoft gives IE access special APIs absolutely necessary for building a modern browser that it won't give to other browsers so there's no way another browser can possibly compete with IE in terms of features or performance.

Anderson and Dotzler said this violated a 2006 statement of principles (PDF) by Microsoft on choice, opportunity and interoperability, and are calling on Microsoft to live up to these principles.

Chrome-maker Google has thrown its weight behind Mozilla. In a statement to The Reg, a spokesperson said the search giant shared Mozilla's concerns about Windows 8's restrictions of user choice and competition.

Google's spokesperson said:

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.

Microsoft's decision to lock down Win RT is not news: introduced as WOA in February, Windows chief Steven Sinofsky said WOA would only support a small number of existing Microsoft apps – Word, Excel, PowerPoint and OneNote with Internet Explorer 10. x86 apps would not run on WOA. WOA would be populated with apps via the Windows Store.

The reason for this was simple and came down to chipset and interface: ARM doesn't support native x86 apps while Windows 8 introduces the tiled and touch-based Metro UI.

For this reason, Microsoft has three development scenarios for Windows 8: Classic - Windows 7-style 32-bit APIs that won't work with Metro; Metro that lives in a sandboxed environment; and "Metro-style enabled" desktop apps that straddle Classic and Metro and call both sets of APIs.

Back in March, Mozilla said it would build a version of Firefox in this third category, with the same system-level parity as IE10, built using traditional Win32 calls and the Windows Runtime WinRT framework that Microsoft has devised for building Windows 8 Metro apps.

At the time Dotzler said: "We should be able to build a single product, that when installed into the Classic environment via traditional means – a download from www.mozilla.org – will be able to become both the default browser in the Classic environment and in the new Metro environment."

At the same time we at The Reg said while this sounded fine in theory, the two big questions would be whether Microsoft would actually permit rivals' browsers to install as the default on Windows 8 and whether Mozilla's work could be transferred to ARM.

Between then and now an answer has been delivered, and it seems this was the catalyst for Mozilla's outburst. In a report here, Anderson says senior Microsoft lawyer David Heiner told him other browsers would not be allowed on ARM. Whether this is a technical or political choice is unclear, although Anderson's implication is there are no technical hurdles.

Dotzler, who had been upbeat in March, now says:

Microsoft has made it clear that the third category won't exist on Windows for ARM (unless you're Microsoft) and that neither will the first category (unless you're Microsoft.) That means that IE on ARM has access to win32 APIs – even when it's running in Metro mode, but no other Metro browser has that same access. Without that access, no other browser has a prayer of being competitive with IE.

Microsoft supporters may dismiss Mozilla and Google, and feel that Redmond is being given a hard time over openness and freedom to install software while others, mainly Apple, enjoy a free pass; Apple also only allows one native browser on its platform for tablets and phones: its own, Safari.

Mozilla, however, feels frustrated because it sees a potential market slipping away by not getting Firefox on Windows RT. ARM dominates smartphones and while it's tiny in tablets today, it has big growth plans. This is the market Mozilla fears losing out on should it expand.

That said, Windows RT is unproven and little known commodity, so there's no telling how successful it will actually be beyond Microsoft's own infectious predictions – that Windows on ARM is its riskiest bet and biggest change in 30 years. We don't know what devices will run Windows RT – although we imagine it will be ereaders; how many types of device there will be; or how they will stand up to the competition from Apple or Amazon.

Microsoft was unable to comment at the time of going to press.®