According to San Diego-based WebSideStory, Firefox now owns 6.75 percent of the American browser market, an increase of more than a full percentage point since the last time the company published its numbers in late February. Internet Explorer, meanwhile, fell to 88.8 percent in April from 89.8 in February.
Less than 12 months ago, IE owned 95.5 percent of the U.S. browser market.
"Firefox marches on," said Geoff Johnston, an analyst with WebSideStory. "About a year ago, when we first started tracking usage, it was a shock that Microsoft's numbers were going backwards. That was something we'd never seen. At first we, and everyone else, thought it would be temporary. But it's not temporary at all.
"I think that with the current rate of increase in Firefox's share -- about a percentage point per quarter -- Firefox will be close to 10 percent in the U.S. by the end of the year," Johnston predicted. "Worldwide, it'll be there for sure."
Firefox's usage share has essentially doubled since June 2004, said Johnston. "Although the recent increases are slightly off the trend of about half a percentage point a month, what's important is that Firefox adoption hasn't leveled off. The groundswell of support really hasn't slowed."
Interesting enough, added Johnston, the U.S. isn't the leader in Firefox fever. In Germany, for instance, 22.6 percent of Internet users hit the Web with Firefox. Under 70 percent, meanwhile, rely on Internet Explorer.
That's in line with Germany's reputation as a haven for open-source and a nation with a decidedly anti-Microsoft tilt. "It makes sense," said Johnston. "They were the very last to finally give up on Netscape. Once the number one stronghold for Netscape during the browser wars, Germany jumped on the Firefox bandwagon first."
Japan, on the other, hand, has essentially ignored Firefox. WebSideStory's numbers show that only 2.8 percent of that country's users run Firefox, compared to 93.9 percent who use Internet Explorer.
Other browsers that WebSideStory tracks include Mozilla and Netscape, both from the same Mozilla Foundation that produces Firefox. The numbers for those open-source tools dropped again, and now account for only 2.2 percent of U.S. usage.
"Firefox doesn't seem like it has any coat tails at all," said Johnston. "Opera, for instance, just hasn't made any traction." Although WebSideStory's "Other" category increased slightly from 1.9 percent in February to 2.06 in April, Johnston said that most of that boost was due to more use of Apple's Safari, not Opera.