Good news for Firefox fans: Microsoft has invited Firefox developers to attend the Windows Vista Readiness ISV Lab in December, to help make sure that Firefox and the Thunderbird email clients work properly under Vista.

No word yet on whether Firefox developers have accepted the invitation. But whether they do or not, this is a good sign from Microsoft.

To put it mildly, Microsoft hasn’t always been particularly amenable to open source software. At times, it’s been downright hostile. That’s been to its detriment; open source software is here to stay, and it’s extremely valuable, for enterprises and home users alike.

Microsoft has reasons to fear Firefox; the browser has gradually been taking away market share from Internet Explorer since its release in 2004, and is now estimated to have 15 percent share of browser usage worldwide, while Internet Explorer has sliped from the high 90s to under 80 percent.

So holding out the peace pipe to Firefox is a particularly piece of good news. In fact, Microsoft has good reason to thank Firefox. Until Firefox came along, Internet Explorer had stagnated, and did’t even have basic features such as tabs. Firefox beat it feature for feature, grabbed market share — and suddenly Microsoft woke up and began serious work in IE.

The result, IE 7 both for XP and Vista, is a fine piece of work. Without Firefox prodding Microsoft, I’m not sure the company would have produced such a good browser.

Let’s hope that asking Firefox into Redmond means even better software, both from open source makers, as well as from Microsoft.