Amsterdam based analytics firm OneStat is reporting that Firefox has captured a worldwide market share of nearly 13%, up from 8.7% in April 2005.

There are some surprising country-by-country statistics included in the report as well. Firefox usage in the U.S. stands at 16%. Australia, 24%. And in Germany Firefox commands a whopping 39% market share.

OneStat is an analytics service for 50,000 websites in 100 countries – the statistics on Firefox are based on aggregate data from visitors to those websites. In the past, statistics from competitor WebSideStory have roughly tracked those from OneStat, although WebSideStory has not released Firefox data since January 2006.

Flock, another Mozilla based browser (and also the browser that I personally use) that recently beta launched publicly and is starting to put distribution deals in place, is aggregated into total “Firefox” usage.

As a non-statistically relevant aside, about 60% of TechCrunch readers use Mozilla based browsers.