Mozilla's Firefox browser has reached a new milestone, exceeding 20 percent worldwide market share for two weeks during October, according to data released by Net Applications. (Via VentureBeat.) During the past year, Microsoft's Internet Explorer has slipped from more than 78 percent of the market to less than 72 percent, according to the data.

The new numbers continue a trend that dates back to the period after the Internet Explorer 6 release. Microsoft, then with a comfortable lead in the market, let IE development go dormant -- opening the door for Firefox to make a big run. Microsoft has since released Internet Explorer 7, and it's now working on IE 8, but Firefox has made improvements of its own and has continued to chip away at IE's lead. Apple's Safari browser, now available on Windows in addition to Mac OS X,  has also been gaining ground over the past year.

Google's Chrome browser, Netscape and Opera each have less than 1 percent of the market.