Internet Explorer dropped under the 70 percent mark last month for the first time since web metrics vendor Net Applications started keeping tabs on browsers, the company has said.
IE slipped to a 69.8 percent share, down from October's 71.3 percent and off 7.6 percentage points in the last year.
Rival browsers from Mozilla, Apple and Google, meanwhile, cashed in on IE's slide and posted gains for the month, according to Net Applications' data, which is culled from visitors to the thousands of websites the company monitors for clients.
Mozilla's Firefox, which briefly popped above the 20 percent share bar during October, solidified that surge in November to end the month at 20.8 percent, an increase of 0.8 percentage points, the largest one-month increase since March 2007.
Apple's Safari and Google's Chrome also gained in October, with Safari accounting for 7.1 percent of users - up 0.6 percentage points - while Google climbed just 0.1 points.
Vince Vizzaccarro, Net Applications' executive vice president of marketing, connected IE's slide - and Firefox's and Safari's impressive increases - to a pair of factors.
"The more home users who are online, using Firefox and Safari at home rather than IE, the more those browsers' shares go up," he said. With November including the Thanksgiving holiday in the US and more weekend days this year - 10 such days versus an average of 8.7 per month - users were at home more than usual.
Net Applications' data has consistently shown that Firefox and Safari usage climbs on weekends and after work hours, as users surf from home computers rather than from work machines, which typically run Microsoft's IE.
"I expect Firefox and Safari to go up in December, too," said Vizzaccarro, citing the higher-than-average number of at-home days for most people this month.
The other factor, he said, is the continued climb in Firefox's share. In the last two years, for example, Firefox's usage share has grown 48 percent, and although there have been times when its growth has stalled, it has posted relatively steady increases.
Safari has also consistently gained share, although not at the same pace as Firefox. Last month's increase, however, was the biggest in the last two years for Apple's browser.
Google's Chrome, which came out strong in early September but later faded, closed November with 0.83 percent, up a bit from October's 0.74 percent. Chrome remains in beta, and is still available only for Windows XP and Vista.
Of the top four non-Microsoft browsers, only Opera's flagship dropped in share during November; it fell to 0.71 percent from October's 0.75 percent.
While Vizzaccarro praised Microsoft's still-in-development IE8, calling it a "pretty nice browser," he wasn't optimistic that it's arrival would turn the tide. "Because IE is used much more in corporations, and they're slower to change, IE8 won't have the same impact as a major Firefox or Safari release," he said.
Microsoft recently said that it will launch IE8 in 2009, sometime after a "release candidate" build is made available during the first three months of next year.
And don't forget unemployment, Vizzaccarro said, arguing that with more people out of work, it will be even tougher for IE to regain ground. "You have to factor for the unemployment rate, too," he said. "That's put a lot more users at home as well, which means more users using Firefox and Safari, not IE."
According to statistics from the US Department of Labor, the unemployment rate in the US rose from 6.1 percent to 6.5 percent during October. Last month's numbers are due out at the end of this week.