Mozilla's Firefox climbed above 20 percent market share for two weeks last month, but ended October just under the milestone, a Web metrics company said Tuesday.

During two separate weeks last month, Firefox exceeded 20 percent share, said Net Applications. For the week ending October 5, the open-source browser averaged exactly 20 percent, while for the week ending October 26, Firefox accounted for 20.06 percent of the browsers used to connect to the thousands of sites Net Applications monitors for clients.

For the month as a whole, however, Firefox's share averaged a close-but-no-cigar 19.97 percent, a jump of nearly half a percentage point over September. That month, Firefox lost ground because of the introduction by Google of its own browser, Chrome. Firefox has now regained what share it lost, and then some.

In June, Vince Vizzaccaro, Net Applications' executive vice president of marketing, had predicted that Firefox might reach the 20 percent mark as early as July, but instead the browser's growth stalled, then slid, when Google released Chrome.

Microsoft's Internet Explorer (IE), meanwhile, lost share again last month. Although IE remains the most-used browser by far, with a 71.3 percent share for October, that was down from 71.5 percent the month before. IE has dropped 7 percentage points in the last year, the bulk of it lost to Firefox, with smaller portions going to Apple's Safari, Opera Software ASA's Opera and Google's Chrome.

Chrome, which came out strong in early September but faded later that month, closed October with 0.74 percent of the browser market, slightly off September's 0.78 percent. Google has released three versions of the Chrome beta so far.

Apple's Safari, which was the only major browser to boost its share in September -- largely because of the lack of a Mac OS X edition of Chrome -- dipped during October to 6.57 percent from 6.65 percent the month before. Apple's share of the operating system market, as measured by Net Applications, also dropped slightly during October.

Mozilla users continued to update from the older Firefox 2.0 to the newer Firefox 3.0 during October; the latter now accounts for 73 percent of all copies of Firefox in use. Mozilla first offered users the upgrade to 3.0 in late August, but today said it plans to repeat the offer later this month to nudge Firefox 2.0 users one more time.

Firefox 2.0 is slated to slip out of support before the end of the year. Mozilla's current plan is to release a final security update for the older browser in mid-December, according to notes from a company status meeting held Monday.

Net Applications' browser market share data is available online.