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One of the most divisive aspects of Microsoft’s Internet Explorer 9 release this week was the company’s decision to support only Windows Vista and Windows 7 with the new browser. That means it doesn’t work on the older Windows XP operating system.

Plenty of people are defending Microsoft’s move — including tech media stalwarts PCWorld and Ars Technica. After all, Windows XP is almost 10 years old. Ten years old! Its moldy security protections are rivaled only by its inability to support some of the most modern graphics technologies.

Clearly, it’s in the interests of Microsoft’s business to get Windows XP users to upgrade to new Windows versions. Leaving them in the cold on the new Internet Explorer is one way to prod them in that direction. But it’s also not an unreasonable suggestion. A decade is more like a century on the web.

[Related Post: Firefox 4 Do Not Track: How it works, what it really means]

Here’s the main problem: A bunch of us haven’t gotten the message. More than 40 percent of web users are still on Windows XP. And that’s the first thing Firefox engineering director Johnathan Nightingale brought up when I asked him about the issue. In an ironic twist, Mozilla’s upcoming Firefox 4 is supporting not only Microsoft’s Windows XP but also Windows 2000, even though Microsoft’s new browser doesn’t.

“That’s a decision that they get to make, but it sure did surprise us, because the best metrics that we’ve got say 40 to 50 percent of the web is still on XP. That’s too big for us to just leave them behind,” Nightingale said via phone. “Yeah, it’s harder work, because XP has a totally different hardware-acceleration story. You need to be using Direct3D there, instead of Direct2D on Vista and 7. It meant we had to do a lot more work architecturally to make sure we could offer a high-quality experience across both of them. But you know what? That’s the job.”

[Follow-up: Firefox 4 soars, thanks to Microsoft’s luddite customers]

This might seem counterintuitive, but the open-source project’s decision to support Windows XP actually helps advance the web, he said.

“It’s not just that we want Firefox to look good on every platform. It’s that, if we don’t do it, web developers aren’t going to take advantage of it. If they can only address 20 percent of the web, they’re just going to sit on their hands for another couple of years and they’re going to build more native apps to do things that the web is perfectly capable of doing, because they just feel like most of their users aren’t going to be able to take advantage of it.”

Here’s the official word from Microsoft on its decision, via a company spokesperson …

“As the Web has continued to change in everything from security to the future HTML5 applications developers are starting to build today, browsers should require the modern graphics and security infrastructure that have come along since 2001. Internet Explorer 9 is intended to be run on a modern operating system in order to build on the latest hardware and operating system innovations. In addition, Web developers can now take advantage of the Windows 7 features (snap, jump list, pinning) integrated into Internet Explorer 9 that make the web feel native. Windows XP users have a fast, safe, reliable and private browser in Internet Explorer 8.”

Which philosophy will prevail? We’ll find out starting next Tuesday, March 22, when Firefox 4 is due out, absent any show-stopping last-minute bugs.

Todd Bishop of GeekWire can be followed on Twitter and Facebook, when he’s not looking longingly at the dusty Windows XP box in the corner of his office.

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