These statistics do not show Firefox is the most popular browser as they are specific only to W3Schools which is primarily a developer/designer-trafficked site.

  The statistics above are extracted from W3Schools' log-files

I'd recommend looking at StatCounter's chart of current browser stats (for the US) as of July 2009 (found this earlier today via TechCrunch): http://gs.statcounter.com/#browser_version-US-monthly-200807...