Mozilla's Firefox keeps chipping away at Microsoft's massive lead in browser usage, two Web metrics firms reported Monday.

San Diego, Calif.-based WebSideStory, which last released usage numbers in January, said that in the last five weeks, Firefox has gained an additional 0.74 percent to account for 5.7 percent of all browsers used in the U.S. Microsoft's Internet Explorer, meanwhile, now stands at 89.9 percent, a drop from January's 90.3 percent, and the first time WebSideStory pegged IE as falling under the 90-percent mark.

"That 7/10s of a point compares well with previous increases," said Geoff Johnston, an analyst for WebSideStory. "From June to November [2004], Firefox saw a pretty steady half-a-percent-point increase each month."

While there are indications that Firefox's growth rate is slowing -- the latest numbers show a growth of about 15 percent over the previous period, while that period had grown by 22 percent over the one before that -- Johnston saw it differently. "I don't see [the growth] stopping or flattening," he said.

"The buzz around Firefox keeps growing, and it's buzz that drives people to change," Johnston added. "Mozilla's marketing has been very guerrilla. You're not seeing it spend millions of dollars on TV. Instead, it's a powerful ground swell."

"Firefox keeps plugging away with new features," said Johnston, of Firefox. "That's what got them where they are today. Microsoft's lack of development on IE left open a window of opportunity, and Firefox took advantage of it. Microsoft didn't think that window was there, but with the announcement that it's working on IE 7.0, they've realized that they had left the window open."

Other browsers, such as Mozilla and Netscape, accounted for 2.5 percent of the apps used, with the "Other" category, which includes Opera and Apple's Safari, made up 1.9 percent, WebSideStory also reported.

When only Windows versions of the browsers are considered, however, WebSideStory's numbers look better to Microsoft. Then, Internet Explorer keeps its numbers above the 90-percent mark, with 92.2. Firefox owns 5.5 of the Windows market, Netscape/Mozilla 2.1 percent, and Opera a mere 0.2 percent.

"Microsoft really only cares about the Windows numbers," said Johnston. Two years ago Microsoft announced it would halt development of its only other version of IE, the one for the Mac, after Apple released its own Safari browser.

Another Web usage measuring firm, OneStat, reported slightly different data, and called Firefox a bigger winner. According to its numbers, Firefox owns 8.5 percent of the browser market, IE has dipped to 87.3 percent, and Safari, Netscape, and Opera each account for between 1.1 and 1.2 percent.

"It seems that global usage share of Mozilla's Firefox is still increasing and the total global usage share of Microsoft's Internet Explorer is still decreasing," said Niels Brinkman, a founder of Amsterdam-based OneStat.com, in a statement. "It looks like that browser users of Internet Explorer 5 are switching to Mozilla Firefox instead of upgrading to Internet Explorer 6.0," he added."

WebSideStory's Johnston said the differences in data were due to the European skewing of OneStat. "They're heavily weighted toward European customers," he said, "just like we are toward U.S. clients. Firefox is clearly doing even better in Europe than here."

With Firefox continuing to nibble at IE, the Mozilla Foundation's goal of reaching a 10 percent market share by the end of the year looks likely, said WebSideStory. "Back in December 2004, it seemed Firefox was a lock to reach 10 percent by mid-2005, ahead of the year-end goal," said Jeff Lunsford, WebSideStory's chief executive, in a statement. "Given the latest growth rates, the year end target still appears attainable, but a mid-year achievement is unlikely unless we see increased marketing activity from Mozilla."

"I don't know how attainable that 10 percent mark is," agreed Johnston. "It's still not clear. What I do know is that there are a solid 10 percent of Internet users who would like to have an alternative to IE."

If even squeezing 10 percent out of IE is tough, it'll be impossible for the Mozilla Foundation to unseat Microsoft entirely. "Mozilla just doesn't have any of the benefits that Microsoft has, like owning the operating system," he said.

"The only way I see Firefox really cracking the big time is if Mozilla partnered with the likes of Google. It's amazing how long it's taken everyone to realize that search is the killer app of the Internet, that search is where the money is for browsers."

Rumors of a Google-Mozilla alliance have been swirling for months, with some recent moves -- particularly the hiring of Firefox developers by Google -- sending the mill into overdrive. For its part, Google has repeatedly denied that it's creating a branded browser.