That didn't surprise one analyst, who cited pent-up demand for Firefox because of several months' worth of delays, and the difference between Firefox and IE9 users.

"The Firefox user base is primarily power user, with a much smaller percentage of corporate users compared with IE9," said Al Hilwa, an analyst with IDC. "Most large IE9 corporate users [work in] lock-downed environments, and so upgrading is under IT control and, thus, slower."

The advantage also goes to Firefox 4 because it runs on Windows XP, the 10-year-old operating system that IE9 has left behind. "Supporting XP will ultimately lead to improving share for Firefox 4 as HTML5 proliferates and users with XP machines want to participate in that," Hilwa said.

According to Web metrics company Net Applications, XP currently accounts for more than 61% of all copies of Windows in use.

Unlike IE9, Firefox 4 also runs on Mac OS X and Linux.