"OneStat.com specifically cited Mozilla's Firefox browser as one reason for the usage shake-up. The Mozilla open-source project released Firefox 1.0 earlier this month."
OneStat.com certainly feels that people are indeed switching from IE to Firefox, which has now reached 5 million downloads since launch. The news seems to have concerned Microsoft on some level, who recently touted the wide range of add-on software available for Internet Explorer that gives users new features, including the tabbed browsing which makes Firefox so popular. This is seen by some as a response to Firefox's growing popularity.