A former Mozilla engineer who worked on the "Metro" version of Firefox argued yesterday that poor adoption of Windows 8's radical user interface (UI) was not the real cause of the decision to shelve the browser.

"Is Windows 8.1 'Modern' UI in trouble? No," said Brian Bondy, who led the Metro-ized Firefox development, in a post on his personal blog.

Bondy's "Modern" is another label for what Microsoft originally called "Metro," the colorful, tile-based, flat-style UI that's one of two in Windows 8 and its follow-on, Windows 8.1. The other is a more traditional desktop UI, relatively familiar to the hundreds of millions who rely on Windows 7, Vista or XP.

A week ago, Mozilla abruptly canceled the release of its touch-enabled Firefox browser for Windows 8 and 8.1, blaming the operating system and its Metro mode.

"We've been watching Metro's adoption," said Johnathan Nightingale, vice president of Firefox, in a March 14 blog. "From what we can see, it's pretty flat. On any given day we have, for instance, millions of people testing pre-release versions of Firefox desktop, but we've never seen more than 1,000 active daily users in the Metro environment."

Nightingale said that the small number of users hadn't been enough to properly test Firefox on Metro, which meant that bugs would invariably slip through the cracks and create massive work for Mozilla after its debut.

Bondy, who last week said he had left the company to join Khan Academy, defended the decision to shelve Firefox but contended it was not because of low interest and usage of the Metro UI.

"Modern UI Firefox usage, in Mozilla's measurements, is not necessarily a true reflection of Modern UI usage in general," Bondy asserted.

Instead, he blamed Microsoft's arcane rules for third-party browsers on Windows 8 and 8.1.

"Microsoft doesn't allow your browser to run in Modern UI unless you are the default browser," Bondy said. "Several people could have had Modern UI capable Firefox pre-releases installed, but just never knew it."

Making a browser the default in Windows 8 and 8.1, and thus available in the Metro UI, is harder than it should be, Bondy continued. "Before Windows 8, each browser could prompt you, and then they could set your default for you. As of Windows 8 you need to ask first, then tell Microsoft to show a prompt that shows a list of browsers," Bondy wrote.

Microsoft's rules for Metro browsers has long been panned by Mozilla, which complained in early 2012 that the Redmond, Wash. company had dragged its feet on offering information about how to craft one.

Microsoft decided that it would allow hybrid desktop-Metro apps, but limited that third category -- after classic x86/64 Windows programs and Metro-only applications -- to something it then called "Metro style enabled desktop browsers" and now labels with the cumbersome "new experience enabled desktop browsers" in a Word document on its support site.

Metro-style desktop browsers were allowed to run outside the normal Metro sandbox and access most classic Windows APIs (application programming interface), as well as the new WinRT API, the backbone of the Metro side of Windows 8 and 8.1 app development.

The category got an important pass from Microsoft: A Metro enabled desktop browser did not have to be distributed through the Windows Store, but could be packaged with its companion for the "Classic" desktop. The caveat, as Bondy noted, was that only the default browser could run in the Metro UI.

Like Windows 7, Windows 8 and 8.1 assign Internet Explorer (IE) as the default browser.

Even so, Mozilla kept to its plan to create a Metro version of Firefox. Two years ago, Bondy said it was "extremely important" to deliver the browser on Metro because he, Mozilla and many others believed that Windows 8, with Microsoft's OS leadership position, would be a success.

Mozilla thought even less of Microsoft's rules about browsers on Windows RT, the touch-only sibling to Windows 8. In May 2012, Mozilla accused its bigger browser rival of anticompetitive behavior by barring third-party browsers from Windows RT, which was designed to run on ARM processor-powered tablets.

In the end, Microsoft did Mozilla a favor. By preventing Mozilla from creating Firefox for Windows RT, Microsoft likely kept Mozilla from going down another dead end, as Windows RT has struggled for any kind of uptake, with most OEMs (original equipment manufacturers) quickly deciding not to support the OS.

Yesterday, Bondy encouraged Microsoft to modify its Metro browser rules. "It would be great if Microsoft could fix these issues around default status. More competition leads to better software, and having good software on your platform is important. Every Windows Modern UI user loses when there's only one Modern UI browser choice," Bondy said.

Microsoft is unlikely to change its mind. Although it has retreated from some of Windows 8's UI choices, first with last year's Windows 8.1 and next month, with the expected Windows 8.1 Update, it has said nothing about abandoning Metro's default browser requirement.

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, on Google+ or subscribe to Gregg's RSS feed . His email address is gkeizer@computerworld.com.

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