Firefox, the free open-source Web browser from Mozilla.org, quietly gained enough users in March to finally grab 10% of the Web browser market, according to a report released Monday by Web audience measurement firm Net Applications.Firefox was used by 10.05% of Web surfers last month, up from 9.75% in February. That compared to the 84.7% of users on various versions of Microsoft Corp's Internet Explorer (IE) in March.In third place with 3.19% was Apple's Safari browser, which used only on Mac platform computers, according to Net Applications, which is based in Aliso Viejo, Calif. and specializes in tracking browser usage at small and midsize businesses.In the past year, Firefox has gained 3.34% share while introducing version 1.5 of the browser with improved features. Internet Explorer has lost 3.89%, as users wait for IE7, which boasts improved features and security and is due out this fall."With a strong feature set, aggressive product development cycle, open source platform and by offering the browser for free, Firefox has the strategy in place to maintain its growth," Vincent Vizzaccaro, an analyst with Net Applications, wrote in yesterday's report. But Microsoft "has the much easier task of defending an entrenched position rather than attacking one."Netscape was the fourth most popular Web browser, with 1.05% of the market, followed by Opera with 0.54% and Mozilla with 0.34%.