Mozilla on Friday abruptly canceled the release of its touch-enabled Firefox browser for Windows 8, just four days before it was to ship and after two years of work.

Firefox for Windows 8 Touch, the browser destined for Windows 8's "Metro" user interface (UI) -- the part of the OS that relies on colorful tiles, mobile-style apps and touch -- will not be released March 18 as part of Firefox 28 as originally planned.

And in a strong signal of Microsoft's struggle to convince customers that Windows 8 is the right OS for the times, Mozilla blamed the operating system and its Metro mode.

"We've been watching Metro's adoption," said Johnathan Nightingale, vice president of Firefox, in a Friday 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."

Those anemic numbers haven't been enough to properly test Firefox on Metro, Nightingale continued, which meant that bugs would invariably slip through the cracks. "That's going to mean lots of bugs discovered in the field, requiring a lot of follow-up engineering, design, and QA effort. To ship it without doing that follow-up work is not an option," he said.

And with such low turn-out for the Metro Firefox, what was the point?

"When I talk about the need to pick our battles, this feels like a bad one to pick: significant investment and low impact," said Nightingale.

The cancelation of Firefox on Metro put a match, more or less, to two years of work by Mozilla's engineers and designers, although Nightingale said that the code would be mothballed, available at some later date if Metro suddenly got a growth spurt.

Mozilla began work on the Metro edition in March 2012. It shipped a preview in October 2012, several weeks before Microsoft launched Windows 8. At that time, Mozilla's schedule said the Firefox app might appear as early as January 2013. But the ambitious timetable was revised several times, until release plans finally firmed up and the March 18 date was nailed down.

Ironically, just as Mozilla was set to ship Metro Firefox, Microsoft was preparing a second round of changes that deemphasize the touch UI. Windows 8.1 Update, a follow-on to last year's Windows 8.1, will likely launch April 8. Although Microsoft has repeatedly reaffirmed its commitment to Metro, the retreat from its original strategy -- to force the UI on users -- may have spooked Mozilla.

Nightingale pointed out that Mozilla had to carefully choose where it commits to compete and where does not. "We still need to focus on the projects with the most impact for our mission; the massive scale of our competitors and of the work to be done requires us to marshal our forces appropriately," Nightingale said.

Of the major browser makers -- Apple, Google, Microsoft, Mozilla and Opera Software -- only Opera recorded less revenue than Mozilla in 2012, the last year that figures were available for all five. And Mozilla has dedicated considerable resources to Firefox OS, the browser-based mobile operating system that it hopes will put it at the smartphone table. Mozilla cited Firefox OS development expenses as one reason why it's considering pushing advertisements to new users of Firefox on the desktop and why it's handed its browser identity project to volunteers.

Even so, pulling the plug on Firefox Metro was a stunning turnabout for Mozilla, which two years ago said that it had play to win. "It is extremely important that we deliver an awesome Firefox experience on Metro, one that is tightly integrated with the platform, fast, and feature rich," wrote Brian Bondy, the Mozilla engineer who led the Metro development team, in April 2012, six months before Windows 8's debut. "Windows is by far the platform with the most users and which has the biggest effect on market share."

The fallout from the decision has begun: Also on Friday, Bondy, who worked at Mozilla since 2011, announced he had left the company and was joining Khan Academy, an online education firm.

Nightingale echoed Bondy's 2012 expectations Friday, but in hindsight.

"In late 2012 ... [Windows 8] looked like the next battleground for the Web. Windows is a massive ecosystem and Microsoft pushes its new platforms hard," said Nightingale.

Instead, while Microsoft has claimed it has sold 200 million Windows 8 licenses and even backed off the "let-them-eat-Metro" strategy, users have continued to pan the Metro UI, the quantity and quality of Metro apps, and Microsoft's decision to mash two UIs into one OS.

Mozilla's move will be seen a vote of no confidence in Windows 8, even though some at Microsoft may welcome the news because it leaves Internet Explorer with only Chrome as a Metro rival.

"Shipping a 1.0 version [of Firefox on Windows Touch], given the broader context we see for the Metro platform, would be a mistake," said Nightingale.

Ouch.

Mozilla has shelved Firefox on Metro because few use the touch-based UI in Windows 8.

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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