Mozilla's popular Firefox browser was downloaded for the 400 millionth time, last Friday.  The company made a celebratory announcement on its website, which can be found here.

Work on the browser was first started on November 9th, 2004, and within 99 days the first version of the browser was released. The browser's latest release version is 2.0.0.6.

As of September 2007, the browser had a 17.4 percent market share, according to marketing research firm Janco and the IT Productivity Center.  This is up from a share of 11.75 percent last year, according to the firm.

In the same period, Internet Explorer's market share dropped from 73.4 to 63.9 percent.  Interestingly, usage of Apple's safari browser also dropped from 1.12 to 1.05 percent.  Google Desktop made a minor splash taking 2 percent of the market away from IE.

Firefox's growth can be attributed to its popularity and an easy to use, intuitive browser. The browser includes tabbed browsing and support for extensions -- two features that many of its users find impossible to live without.  While other browser support tabbed browsing, Firefox is the clear leader in extensions support, as most of its competitors provide little if any support for extensions.