You’d think we’d be finished with all the nasty antitrust legal issues surrounding computer operating systems by now. Windows is still powerful, but it’s a shadow of its former monopolistic self, and Mac OS X, iOS, Android, and Linux are all viable, strong, healthy competitors in various niches of the computing ecosystem.

But not according to the European Union. And, not according to Mozilla or Google.

At issue, according to an article posted by ComputerWorld, is secret APIs.

APIs, or application programming interfaces, are used by applications to plug in functionalities on a computing device or service. In an operating system, that might mean access to the file system, graphical outputs to a screen, or the ability to create and manage windows in an application.

Secret APIs are APIs that the owner of a system creates but does not share with partners or developers, giving the ecosystem creator potential advantages in application development. That’s exactly what Microsoft stands accused of doing.

“Internet Explorer 10 on Metro has special access to some very powerful APIs from over in Win32 land,” Asa Dotzler, a Mozilla spokesperson, writes in a personal blog post. Those APIs enable Microsoft’s browser to run quickly and efficiently on Javascript-intensive websites.

Unfortunately, however, non-Microsoft browsers on ARM processors are not getting the same treatment, Dotzler alleges. “Microsoft is giving its own Internet Explorer special privileges that no other Metro app, including other Metro browsers, are allowed.”

The upshot, according to Dotzler:

If we built Firefox for Windows ARM Metro, we would not have access to those powerful Win32 APIs and so we would be at an extreme disadvantage when compared to IE 10 for Metro. We could build a beautiful Firefox that looked really nice on Metro, but Firefox would be so crippled in terms of power and speed that it’s probably not worth it to even bother. No sane user would want to surf today’s web and use today’s modern websites with that kind of crippled browser.

Google will, of course, have similar concerns with its Chrome browser.

The EU will be investigating this claim, according to Reuters, and will be adding this new claim to the already-announced investigation into Microsoft’s alleged failure to provide a browser choice screen in a Windows 7 service page issued in 2011.

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