A few weeks ago, we covered how Mozilla’s Firefox, the open-source web browser, is now used by more than 160 million people around the world. Earlier today, we covered Firefox’s plans to develop a robust mobile browser. Now, here’s some more data for you about Firefox’s growth around the world, based on a study by French web survey company ZiTi Monitor.

This past March, according to the study, Firefox was being used by nearly 29 percent of all European Internet users. Firefox is already more popular in Europe than in the US, where it is used by nearly 22 percent of people.

Notably, Firefox usage went up to a little more than 30 percent on weekends, presumably because people were accessing the web from home, where they could choose it over competing browsers rather than being forced to use the browser selected by the IT manager at their workplace.

This matters for many web companies that at least partially rely on browser plugins to work. While Firefox’s biggest rival, Microsoft’s Internet Explorer, also offers plugins, that browser is generally harder to develop applications and plugins that work well on it. Sample plugin oriented companies include news-sharing site StumbleUpon, phone bill tracker Skydeck and social info aggregator MySocial24x7.

Firefox is gearing up for its next release, version 3, later this year. You can test out the beta version here. Testing it out, we’ve found it to be much faster than Firefox 2 currently is, especially on Apple computers.

Here’s the full study, in French.