Although Microsoft is alleged to refer to it as "the open sores browser" in internal meetings, the Mozilla Foundation yesterday celebrated the 100 millionth download of Firefox.

But while the Mozilla Foundation toasted this achievement with Champagne, and thanked the Firefox community for "reigniting innovation on the web", critics were sceptical of its claimed user base.

Amar Rathore, a Firefox user at Bracknell-based open source software company Countersnipe, told vnunet.com: "I think I make up 10 of its users. Every time I have to update, I seem to register as a new user. So I'm not surprised that Firefox has had 100 million downloads."

The Mozilla Foundation was not entertaining any such negativity, however. " Firefox's download success is a direct result of the collaborative efforts of thousands of contributors worldwide," said a spokesman.

"Their work in developing and fine-tuning the Firefox browser ensures the best web experience available. Volunteer extension developers further enrich Firefox's capabilities by enabling users to customise and enhance their browser and truly take back the web."

After Firefox was released in November 2004 it racked up 50 million downloads in six months. Firefox 1.5 Beta 2, the second test version of the new browser, is due for release before Christmas.

However, industry watchers have reacted with incredulity to the Mozilla Foundation's recently announced plans to launch a weather balloon.

'Firebox One' will be released into the San Francisco smog on Saturday. It will reach an altitude of 100,000ft and release a 14lb payload comprising a large Firefox banner and a CD, along with a camera to take photos of Firefox " at the edge of space", according to insiders.