Microsoft has poured scorn on Mozilla’s claims that its open source Firefox web browser has been downloaded one billion times and asked people to be “somewhat sceptical” about the figures.

Mozilla claimed Firefox had reached the billion-download milestone in July this year, a figure that includes every update made to the browser since its launch in 2004.

According to a report in the Guardian, however, Microsoft’s Amy Barzdukas, general manager of the Internet Explorer web browser, doubts the figures are genuine.

“As with any marketing statement, I’d encourage people to be somewhat sceptical about large number claims,” she said. “It’s an interesting number and I have not seen the math [but] how many internet connected users are there? 1.1 billion, 1.5 billion, something in that area.”

Barzdukas also announced that Microsoft would extend support for Internet Explorer 6, a browser first launched in 2001, until 2014. IE6 is still the most popular browser around, connecting more people to the web than any other, but security issues and the release of IE8 suggested that its lifespan was coming to an end.

IE6’s widespread use in the enterprise is the main reason for its continued existence, Barzdukas said, even though consumer are switching away from it. “Friends do not let friends use IE6. If you are in my social set and I have been to your house for dinner, you are not using IE6. But it is much more complicated when you move into a business setting,” the BBC quotes Barzdukas as saying.

Microsoft’s Dean Hachamovitch backed up these claims in a blog post. “Many PCs don’t belong to individual enthusiasts, but to organisations. The backdrop might be a factory floor or hospital ward or school lab or government organisation, each with its own business applications,” he said. “Dropping support for IE6 is not an option because we committed to supporting the IE included with Windows for the lifespan of the product.”