Net Applications is. It’s right there in the Wikipedia article you are linking to. Net Applications measure unique visits and do country level weighing of data. The article is also quoting Net Application’s desktop numbers. That might explain the difference, at least partly.

Overall I would argue that the statement in the article, while accurate, can be misleading. It should probably include mobile usage numbers (though the sentence you quoted makes clear that they don’t – and on mobile phones Firefox isn’t much of a player anyway) and maybe note that market share measurement of browsers can be unreliable.